requests: allow passing a list of tuples for files

[tommilligan]

Relates to #7724

The files argument in the requests library is incompletely typed. The current types include the Mapping type (over which there has been considerable discourse already), but the official requests documentation states:

You can send multiple files in one request. [...]
To do that, just set files to a list of tuples of (form_field_name, file_info):

The official example code given is:

url = 'https://httpbin.org/post'
multiple_files = [
    ('images', ('foo.png', open('foo.png', 'rb'), 'image/png')),
    ('images', ('bar.png', open('bar.png', 'rb'), 'image/png'))]
r = requests.post(url, files=multiple_files)
r.text

This snippet currently fails typechecking with the latest stubs. A minimal reproduction is available here, which also confirms the code works as expected, but fails with.

minimal.py:15: error: Argument "files" to "post" has incompatible type "List[Tuple[str, Tuple[str, BufferedReader, str]]]"; expected "Optional[Union[Mapping[str, Union[SupportsRead[Union[str, bytes]], str, bytes]], Mapping[str, Tuple[Optional[str], Union[SupportsRead[Union[str, bytes]], str, bytes]]], Mapping[str, Tuple[Optional[str], Union[SupportsRead[Union[str, bytes]], str, bytes], str]], Mapping[str, Tuple[Optional[str], Union[SupportsRead[Union[str, bytes]], str, bytes], str, MutableMapping[str, str]]]]]"
Found 1 error in 1 file (checked 1 source file)

[tommilligan]

A suggested fix for this issue has been opened at #7999