Change 'dict' annotations to 'Mapping'; do not require string annotations from users

[hmgaudecker]

1. Reason: We want to support immutable mappings in pylcm (see issue #136).
2. We can just work with strings internally (and will return strings) while not making assumptions about user code.

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.48%. Comparing base (5893b76) to head (34e892a).

Files with missing lines	Patch %	Lines
tests/test_signature.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##           chore-improve-linting      #67      +/-   ##
=========================================================
- Coverage                  95.57%   95.48%   -0.09%     
=========================================================
  Files                         23       23              
  Lines                       1174     1151      -23     
=========================================================
- Hits                        1122     1099      -23     
  Misses                        52       52              

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[hmgaudecker]

Note: Docs failure is expected, will be fixed in #65