Add support for tuples in get_attr

[SamRemis]

The get_attr function is intended to be able to support an input with a path such as [0] and a value of ['foo']and parse out the fist index of the value to get foo as the result.

Without the change in this PR, the such an input will not match the indexing regex, and instead it will reach the else statement and reach the code intended to access dictionary indexes. This will fail with a type error, so this PR updates the regex to properly reach the part of the code that handles indexes without a word string before them.

That code still fails for the same input; it assumes that it is getting the index of a list from a dictionary, so it attempts to call .get() on the base input. This will not work if the input value is not a dictionary, so we need to add the conditional to only call .get() if there is a string before the first open bracket in the part of the path that we are parsing.

Finally, this code can't be reached currently with a list due to how we memoize input parameters to endpoints; all parameter types have to be hash-able. While this function could theoretically get called with a list as input from another context, this is a private interface and it is only internally called from the endpoint provider. Because of this, I've updated the docblock to specify tuple as the input type instead of list.

[alexgromero]

LGTM

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.18%. Comparing base (b1279af) to head (6fb6221).
Report is 98 commits behind head on develop.

Files with missing lines	Patch %	Lines
botocore/endpoint_provider.py	83.33%	1 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3302   +/-   ##
========================================
  Coverage    93.18%   93.18%           
========================================
  Files           66       66           
  Lines        14345    14378   +33     
========================================
+ Hits         13367    13398   +31     
- Misses         978      980    +2     

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