Use hiredis::pack_command to serialized the commands.

[prokazov]

@chayim @dvora-h

Pull Request check-list

Please make sure to review and check all of these items:

• [x ] Does $ tox pass with this change (including linting)?

• [ x] Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?

• [ x] Is the new or changed code fully tested?

• [ ?] Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

• [n/a ] Is there an example added to the examples folder (if applicable)?

• [ x] Was the change added to CHANGES file?

  Historically, hiredis-py (Python extension module written in C and utilizing hiredis library) has been used only for deserialization of the Redis command's replies. With the new hiredis-py 2.2.1 release it's possible to do the opposite operation - serialization of the Python tuples, representing commands in the redis-py into a RESP-encoded bytes buffer. This PR:

1. Extracts the pack command functionality form the Connection class and delegates it to an external object.
2. PythonRespSerializer class that implements the extracted functionality.
3. HiredisRespSerializer class that uses hiredis-py to serialize a redis-py command.

[codecov-commenter]

Codecov Report

Base: 92.28% // Head: 92.27% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (566712e) compared to base (428d609).
Patch coverage: 90.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2570      +/-   ##
==========================================
- Coverage   92.28%   92.27%   -0.02%     
==========================================
  Files         115      115              
  Lines       29660    29691      +31     
==========================================
+ Hits        27373    27396      +23     
- Misses       2287     2295       +8     

Impacted Files	Coverage Δ
redis/connection.py	86.74% <89.13%> (+0.06%)	⬆️
redis/utils.py	92.85% <100.00%> (+0.26%)	⬆️
tests/test_encoding.py	98.78% <100.00%> (+0.03%)	⬆️
redis/asyncio/cluster.py	91.68% <0.00%> (-0.33%)	⬇️
tests/test_asyncio/test_pubsub.py	99.37% <0.00%> (-0.16%)	⬇️
tests/test_cluster.py	96.75% <0.00%> (-0.11%)	⬇️

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[spicy-sauce]

seems that elif and else are not needed:

if packer is not None:
    return packer

if HIREDIS_PACK_AVAILABLE:
    return HiredisRespSerializer()

return PythonRespSerializer(self._buffer_cutoff, self.encoder.encode)

[prokazov]

Technically - yes, we can get rid of else but , my understanding of general sentiment regarding elif in Python is: it's cleaner way to show that conditions intended to be mutually exclusive.

[chayim]

@prokazov given that hiredis isn't a dependency, but is an extra (i.e. pip install redis[hiredis] we can't enforce a hiredis version on existing users. This means that the hiredis check now becomes more complicated.

We need to wrap the pack_command call, with a check to ensure that hiredis is >= 2.2.1, specifically due to this change, otherwise we'll break the community.

Similarly @dvora-h this becomes 4.5.0 as discussed.

[zalmane]

@chayim Originally the check was checking for a version. It was later changed to check for the existence of the pack_command exposed in the hiredis. So effectively this validates if the hiredis-py contains this new method so it is backwards compatible.

[dvora-h]

LGTM!