Misc. type fixes and improvements

[AdeelH]

Overview

This PR makes various type-related fixes and improvements, plus some minor refactoring. It also adds py.typed files.

Checklist

• Added unit tests, if applicable
• Updated documentation, if applicable
• Added needs-backport label if the change should be back-ported to the previous release
• PR has a name that won't get you publicly shamed for vagueness

Notes

Some of the problems fixed by this PR were surfaced by mypy, but it should be noted that the full codebase is not yet fully mypy-approved.

Testing Instructions

N/A

[codecov]

Codecov Report

Attention: Patch coverage is 89.28571% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.19%. Comparing base (8ab5837) to head (1597673).
Report is 38 commits behind head on master.

Files with missing lines	Patch %	Lines
...ision_aws_s3/rastervision/aws_s3/s3_file_system.py	80.00%	1 Missing ⚠️
...n/core/rv_pipeline/semantic_segmentation_config.py	50.00%	1 Missing ⚠️
rastervision_pipeline/rastervision/pipeline/cli.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2210      +/-   ##
==========================================
- Coverage   90.27%   90.19%   -0.08%     
==========================================
  Files         198      198              
  Lines        9902     9896       -6     
==========================================
- Hits         8939     8926      -13     
- Misses        963      970       +7     

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