introduce AbstractConnection so that UnixDomainSocketConnection can call super().__init__

[woutdenolf]

Pull Request check-list

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

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

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afterwards / while the PR is open.

Description of change

Motivation: changes to Connection.__init__ are easily forgotten to be applied also to UnixDomainSocketConnection.__init__ (for example #2570 and #2377) which leads to issues like #2581.

Before this MR

class Connection:
    ...

class UnixDomainSocketConnection(Connection):
    def __init__(...):
        # does not call super().__init__(...)

After this MR

class AbstractConnection:
    ...

class Connection(AbstractConnection):
    def __init__(...):
        ...
        super().__init__(...)

class UnixDomainSocketConnection(AbstractConnection):
    def __init__(...):
        ...
        super().__init__(...)

[codecov-commenter]

Codecov Report

Base: 92.31% // Head: 92.26% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (2df7b6d) compared to base (6c708c2).
Patch coverage: 78.26% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2588      +/-   ##
==========================================
- Coverage   92.31%   92.26%   -0.06%     
==========================================
  Files         115      115              
  Lines       29744    29729      -15     
==========================================
- Hits        27459    27429      -30     
- Misses       2285     2300      +15     

Impacted Files	Coverage Δ
redis/connection.py	86.21% <78.26%> (-0.58%)	⬇️
tests/conftest.py	84.94% <0.00%> (-0.78%)	⬇️
tests/test_asyncio/test_search.py	98.29% <0.00%> (-0.35%)	⬇️
redis/asyncio/cluster.py	91.68% <0.00%> (-0.33%)	⬇️
tests/test_cluster.py	96.64% <0.00%> (-0.33%)	⬇️

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