Add configurable chain separator (Issue #52 - 1/2)

[rsnk96]

This PR adds a configurable chain separator to RedisDict (resolves #52). Previously, the chain separator was hardcoded to :, which could cause key collisions when user keys contained colons. The separator is now configurable via a constructor parameter and defaults to ➡️ (emoji + spaces) to avoid collisions in typical use cases.

Test code:

from redis_dict import RedisDict

# Create RedisDict with custom separator
r = RedisDict(namespace='test')
r.clear()

# Use chain operations
r.chain_set(['foo', 'bar'], 'value1')
r.chain_set(['foo', 'bar2'], 'value2')

# No collision with user keys containing ':'
r['foo:bar'] = 'direct_value'

print("Normal chain get of foo, bar: ", r.chain_get(['foo', 'bar']))
print("Specific key explicitly set:  ", r['foo:bar'])
print("Multi chain get of foo:       ", r.multi_chain_get(['foo']))
print("List of all keys:             ", list(r._scan_keys("*")))

Behaviour before this PR:

Normal chain get of foo, bar:  direct_value
Specific key explicitly set:   direct_value
Multi chain get of foo:        ['value2', 'direct_value']
List of all keys:              ['test:foo:bar2', 'test:foo:bar']

Behaviour after this PR:

Normal chain get of foo, bar:  value1
Specific key explicitly set:   direct_value
Multi chain get of foo:        ['value1', 'direct_value', 'value2']
List of all keys:              ['test:foo➡️     bar', 'test:foo:bar', 'test:foo➡️     bar2']

[Attumm]

There was a reason for 3.8 and not higher. Can't remember why exactly. Hmm

[rsnk96]

Haha I'm guessing you mean the failing pylama. Well, we'll get that fixed now!