Customize the input buffer size

[asvetlov]

Fixes #4453

[github-actions]

💔 Backport was not successful

The PR was attempted backported to the following branches:

• ❌ 3.7: Commit could not be cherrypicked due to conflicts

[tseaver]

Making limit a required positional argument (removing the default) is a breaking change: see pnuckowski/aioresponses#173

[asvetlov]

Sorry but no. StreamReader.__init__ is a private API, user code should never call it.
aioresponses mocks aiohttp internals for its work.
I cannot prevent it (but discourage, sure).
The breakage of such libraries when aiohttp changes own internal undocumented API is the expected thing unfortunately.

[tseaver]

I'm not sure I understand how the constructor of a public class (aiohttp.streams.StreamReader is even imported into aiohttp/__init__.py!) can be considered "private". I do see the note in the docs that users aren't to instantiate the class, but that is a very unusual claim.

[asvetlov]

Sorry for that