I just realized that the handling of streams this PR introduces is immature. It

• makes Promise assume that every stream is supposed to be converted into in InputFile, isn't necessarily the case for running custom functions through Dispatcher.run_async. Then again, using Dispatcher.run_async with context managers for reading files into custom functions will not work in either case …
• doesn't pass additional arguments to InputFile (or parse_file_input, which will be used after API 5.0 WP4.3: Handling of files in local mode #2223)

Possible ways I see:

1. Just don't handle it. opening files is the only context manager situation we can control in the first place, so it may not be worth making an effort for it. We just document that context managers don't work with promises in general.

2. Try to handle at least file-context managers for bot methods. Sketch of an idea:

   @mq(input_file_args={'media': {'filename': 'filename', attach=True, 'tg_type': Audio}})
   def bot_method(media, filename, *args):
       …

   so that the @mq decorator can do the handling of file input for the specified arguments, looking up values for parse_file_input in the other arguments. Ofc. this leads to some code duplication and is rather hacky.

For the moment I think I'd be fine with 1., i.e. just not handling it.

In fact just not supporting context managers will not lead to any major issues for users once #2230 is implemented as

with open('file', 'rb): as file:
    bot.send_document(chat_id, file)

can then be replaced by

with open('file', 'rb): as file:
    bot.send_document(chat_id, file.read())

Edit: Went for it. The wiki page needs to be updated anyway, so we can just put a note there.

ready for review now right? GitHub already complains about a merge confilct you might wanna solve that ;P

Closing for now as we'll go a different route (see #2139 for further details). Might still be worth revisiting later.