Make Retry type hint more exact

[hongquan]

The current type hint of Retry makes MyPy check failed when user passes a tuple to interval parameter, though that usage is correct.

[selwin]

Mind making the tests pass so I can merge this in? @hongquan

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.88%. Comparing base (2de9491) to head (b11097a).
Report is 73 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2097      +/-   ##
==========================================
+ Coverage   93.61%   93.88%   +0.26%     
==========================================
  Files          28       30       +2     
  Lines        3760     3991     +231     
==========================================
+ Hits         3520     3747     +227     
- Misses        240      244       +4     

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[hongquan]

@selwin The failed test case is not related to this PR and it happens for older stack (Redis server v5 and redis-py v3.5). Could we just drop support for those stack?