Fix errors reported by YAMLLint + use old Ubuntu

Trying older Ubuntu because of https://bugs.launchpad.net/ubuntu/+source/calibre/+bug/1884295

Author: swaroopch

.github/workflows/main.yml

@@ -3,80 +3,86 @@ name: CI
 
 on:
   push:
-    branches: 
+    branches:
       - master
 
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     steps:
 
-    - name: Install Calibre
-      run: sudo apt install calibre  # for `ebook-convert` command
+      - name: Install Calibre
+        run: sudo apt install calibre  # for `ebook-convert` command
 
-    # https://github.com/actions/setup-node
-    - name: Install Node.js
-      uses: actions/setup-node@v2-beta
-      with:
-        node-version: '14'
-   
-    # https://github.com/actions/checkout
-    - name: Checkout
-      uses: actions/checkout@v2
+      # https://github.com/actions/setup-node
+      - name: Install Node.js
+        uses: actions/setup-node@v2-beta
+        with:
+          node-version: '14'
 
-    # https://github.com/honkit/honkit
-    - name: Run Honkit
-      run: |
-        npm install honkit --save-dev
-        npx honkit build . public --log=debug
-        npx honkit pdf . byte-of-python.pdf
-        npx honkit epub . byte-of-python.epub 
+      # https://github.com/actions/checkout
+      - name: Checkout
+        uses: actions/checkout@v2
 
-    # https://github.com/crazy-max/ghaction-github-pages
-    - name: Push to GitHub Pages
-      if: success()
-      uses: crazy-max/ghaction-github-pages@v2
-      with:
-        target_branch: gh-pages
-        build_dir: public
-        fqdn: python.swaroopch.com
-      env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      # https://github.com/honkit/honkit
+      - name: Run Honkit
+        run: |
+          npm install honkit --save-dev
+          npx honkit build . public --log=debug
+          npx honkit pdf . byte-of-python.pdf
+          npx honkit epub . byte-of-python.epub
 
-    # https://github.com/actions/create-release
-    - name: Create Release
-      id: create_release
-      uses: actions/create-release@v1
-      env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-      with:
-        tag_name: v${{ github.sha }}
-        release_name: Release v${{ github.sha }}
-        draft: false
-        prerelease: false
+      # https://github.com/crazy-max/ghaction-github-pages
+      - name: Push to GitHub Pages
+        if: success()
+        uses: crazy-max/ghaction-github-pages@v2
+        with:
+          target_branch: gh-pages
+          build_dir: public
+          fqdn: python.swaroopch.com
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
-    # https://github.com/actions/upload-release-asset
-    - name: Upload Release PDF
-      id: upload-release-pdf
-      uses: actions/[email protected]
-      env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-      with:
-        upload_url: ${{ steps.create_release.outputs.upload_url }} # This pulls from the CREATE RELEASE step above, referencing it's ID to get its outputs object, which include a `upload_url`. See this blog post for more info: https://jasonet.co/posts/new-features-of-github-actions/#passing-data-to-future-steps 
-        asset_path: ./byte-of-python.pdf
-        asset_name: byte-of-python.pdf
-        asset_content_type: application/pdf
-  
-    # https://github.com/actions/upload-release-asset
-    - name: Upload Release EPUB
-      id: upload-release-epub
-      uses: actions/[email protected]
-      env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-      with:
-        upload_url: ${{ steps.create_release.outputs.upload_url }} # This pulls from the CREATE RELEASE step above, referencing it's ID to get its outputs object, which include a `upload_url`. See this blog post for more info: https://jasonet.co/posts/new-features-of-github-actions/#passing-data-to-future-steps 
-        asset_path: ./byte-of-python.epub
-        asset_name: byte-of-python.epub
-        asset_content_type: application/epub+zip
+      # https://github.com/actions/create-release
+      - name: Create Release
+        id: create_release
+        uses: actions/create-release@v1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          tag_name: v${{ github.sha }}
+          release_name: Release v${{ github.sha }}
+          draft: false
+          prerelease: false
+
+      # https://github.com/actions/upload-release-asset
+      - name: Upload Release PDF
+        id: upload-release-pdf
+        uses: actions/[email protected]
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          # This pulls from the CREATE RELEASE step above, referencing it's ID to get its outputs object, which include
+          # a `upload_url`. See this blog post for more info:
+          # https://jasonet.co/posts/new-features-of-github-actions/#passing-data-to-future-steps
+          upload_url: ${{ steps.create_release.outputs.upload_url }}
+          asset_path: ./byte-of-python.pdf
+          asset_name: byte-of-python.pdf
+          asset_content_type: application/pdf
+
+      # https://github.com/actions/upload-release-asset
+      - name: Upload Release EPUB
+        id: upload-release-epub
+        uses: actions/[email protected]
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          # This pulls from the CREATE RELEASE step above, referencing it's ID to get its outputs object, which include
+          # a `upload_url`. See this blog post for more info:
+          # https://jasonet.co/posts/new-features-of-github-actions/#passing-data-to-future-steps
+          upload_url: ${{ steps.create_release.outputs.upload_url }}
+          asset_path: ./byte-of-python.epub
+          asset_name: byte-of-python.epub
+          asset_content_type: application/epub+zip