Summary

This PRresolves#1620 and implements chunked upload foranyio.open_file,trio.open_file andaiofiles.open_file when used ascontent= parameter forpost andput request. And implements support for multipart file upload for the same libraries. Most of the code for multipart file upload copied from my old stale PR#3339.

changes:

• _compat.py file added to defineTypeIs andaclosing for the range of supported python versions
• _types.AsyncReadableFile protocol was added along withis_async_readable_file type predicate function to detect and perform type narrowing for trio/asyncio/aiofiles async files
• _types.FileContent was extended to include the._types.AsyncReadableFile protocol
• _content.AsyncIteratorByteStream updated to useasync read methods for async files instead of looping over lines
• _content.encode_content updated to attach content length header for_types.AsyncReadableFile type
• _multipart.FileField updated to include async versions ofrender andrender_data
• _multipart.MultipartStream updated with async version ofiter_data and__aiter__ method updated to use it
• test_chunked_async_file_multipart andtest_chunked_async_file_content tests are added to test the new functionality
• aiofiles andtypes-aiofiles added in requirements to test aiofiles (can be removed with test if aiofiles support is not desirable)

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly. (can update if PR is ok otherwise)