Update mypy configuration for Python 3.8

[rmol]

Status

Ready for review

Description of Changes

Updates our mypy configuration to expect Python 3.8. We have dependencies using variable annotation syntax, causing Variable annotation syntax is only supported in Python 3.6 and greater errors from make typelint.

Also remove special-case linting of securedrop/rm.py.

Testing

• make typelint should report no issues found.

Deployment

Dev only.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

[codecov-commenter]

Codecov Report

Merging #5977 (b123c09) into develop (59da5b7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5977   +/-   ##
========================================
  Coverage    85.32%   85.32%           
========================================
  Files           53       53           
  Lines         3883     3883           
  Branches       481      481           
========================================
  Hits          3313     3313           
  Misses         457      457           
  Partials       113      113           

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[kushaldas]

This is good, other broken PRs will need a rebase after this is merged.