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 97.72727% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.48%. Comparing base (96ad2ec) to head (1a23d6a).
⚠️ Report is 1 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #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

[timmens]

This is amazing! Makes dags way more practical and easier to use.