What do these changes do?

They adjust the annotations on the MultipartWriter.append method (and two similar methods), so that type-checkers admit plain python dictionaries for the second argument; the headers parameter.

I chose to use LooseHeaders as I knew it was already used as the headers parameter for the web.Response constructor, among others. Changed to Mapping[str, str] , thank you Dreamsorcerer

Are there changes in behavior for the user?

No, this doesn't change any behaviour.

Is it a substantial burden for the maintainers to support this?

Not that I can imagine.

Related issue number

Fixes #7741

Checklist

• I think the code is well written
• Unit tests for the changes exist
  • I think the existing tests should be sufficient for seeing that the str and istr keyed dictionaries work with append

    • aiohttp/tests/test_multipart.py

      Line 1310 in d8936aa

      writer.append("hello, world!", {"x-foo": "bar"})

    • aiohttp/tests/test_multipart.py

      Line 1323 in d8936aa

      writer.append(part, {CONTENT_TYPE: "test/passed"})

      
      but am also open to suggestions
• Documentation reflects the changes
  • Documentation already gives examples

    • aiohttp/docs/multipart.rst

      Lines 145 to 146 in d8936aa

      	mpwriter.append(io.BytesIO(b'GIF89a...'),
      	{'CONTENT-TYPE': 'image/gif'})

• If you provide code modification, please add yourself to CONTRIBUTORS.txt
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  • name it <issue_or_pr_num>.<type>.rst (e.g. 588.bugfix.rst)

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      improper undesired behavior that got corrected to match
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    Fixed issue with non-ascii contents in doctest text files
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