Update repr of important classes with module name and recommended "< …

[kristjanvalur]

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Description of change

the __repr__ of important classes was missing the name of the module, which is often important for
disambiguation. Also, the format should typically be "< ... >", according to the documentation, for consistency.
Update repr and tests accordingly

The motivation for this is to get more informative ResourceWarning errors, e.g. via PR #2999

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (5391c5f) 91.36% compared to head (d2d7304) 77.29%.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3001       +/-   ##
===========================================
- Coverage   91.36%   77.29%   -14.08%     
===========================================
  Files         126      126               
  Lines       32617    32618        +1     
===========================================
- Hits        29802    25212     -4590     
- Misses       2815     7406     +4591     

Files	Coverage Δ
redis/asyncio/client.py	92.13% <100.00%> (ø)
redis/asyncio/connection.py	82.16% <100.00%> (-1.50%)	⬇️
redis/connection.py	79.11% <100.00%> (-3.21%)	⬇️
tests/test_asyncio/test_connection_pool.py	92.03% <100.00%> (+0.01%)	⬆️
tests/test_connection_pool.py	100.00% <100.00%> (ø)
redis/client.py	91.67% <0.00%> (-0.31%)	⬇️
redis/asyncio/sentinel.py	83.92% <0.00%> (ø)
redis/sentinel.py	86.95% <0.00%> (ø)

... and 30 files with indirect coverage changes

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[chayim]

@dvora-h I think this is a 5.1 change if it's the sort of thing we want to consider.

[kristjanvalur]

Sure. the changes are basically:

1. give the module name as well as the class, to avoid ambiguity.
2. Put the entire thing in <> as is the Python convention: https://docs.python.org/3/reference/datamodel.html?highlight=__repr__#object.__repr__

The reason I touched this was to make sure that any ResourceWarning errors would provide as useful info as possible.