Fixes requirements.txt, applies default headers before header params

[spacether]

Fixes requirements.txt (adds missing typing_extension), uses same libs for setup.py

• adds missing typing_extension
  • typing_extensions limited to minor version >= for general safety
  • frozendict limited to minor version >= for test success on dict representation
  • python-dateutil limited to minor version >= for isoparser existence
  • urllib3 limited to minor version >= for supports_chunked_reads
  • Note: other libs can be more constrained in the future as needed
• fixes headers
  • default headers set first then set ones applied on top
  • adds verification tests test_passed_in_header_overrides_default test_default_header_used_when_no_header_params_input

When merged, this will fix:

• [BUG] [PYTHON] Missing 'typing_extensions' in requirements.txt OpenAPITools/openapi-generator#13621
• [BUG] [Python][Template] client default headers override explicitly passed headers OpenAPITools/openapi-generator#9648

And those fixes will need to be merged into openapi-generator at a later date

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