Rectify nearly all type errors revealed by mypy on current repo

[emeryberger]

This PR resolves nearly all of the errors produced by mypy and passes the same number of tests as before, reducing the number of type errors from 43 to 7 (6 of which are in the vendored code from six). This is laying the groundwork for a more extensive effort at annotating this code.

Before:

Found 43 errors in 13 files (checked 74 source files)

After:

Found 7 errors in 2 files (checked 74 source files)

Command line:

% mypy --install-types --non-interactive --ignore-missing-imports botocore

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.18%. Comparing base (0e229ae) to head (c727b5a).
Report is 59 commits behind head on develop.

Files with missing lines	Patch %	Lines
botocore/crt/auth.py	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3267   +/-   ##
========================================
  Coverage    93.17%   93.18%           
========================================
  Files           66       66           
  Lines        14339    14342    +3     
========================================
+ Hits         13361    13365    +4     
+ Misses         978      977    -1     

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