Add SMTP constructor keyword 'stream_reader_class', default asyncio.S…

[davidmcnabnz]

…treamReader, allowing user subclass

What do these changes do?

Allows a caller to specify their own subclass of asyncio.StreamReader

Are there changes in behavior for the user?

This will not affect behaviour for users who do not provide a 'stream_reader_class' SMTP constructor keyword.
But for callers who do provide their own subclass, this gives the
ability to override methods of asyncio.StreamReader, such as .readuntil()

Related issue number

#285

Fixes #285

Checklist

• I think the code is well written
• Unit tests for the changes exist
• tox testenvs have been executed in the following environments:
  • Linux (Ubuntu 18.04, Ubuntu 20.04, Arch): {py36,py37,py38,py39}-{nocov,cov,diffcov}, qa, docs
  • Windows (7, 10): {py36,py37,py38,py39}-{nocov,cov,diffcov}
  • WSL 1.0 (Ubuntu 18.04): {py36,py37,py38,py39}-{nocov,cov,diffcov}, pypy3-{nocov,cov}, qa, docs
  • FreeBSD (12.2, 12.1, 11.4): {py36,pypy3}-{nocov,cov,diffcov}, qa
  • Cygwin: py36-{nocov,cov,diffcov}, qa, docs
• Documentation reflects the changes
• Add a news fragment into the NEWS.rst file
  • Add under the "aiosmtpd-next" section, creating one if necessary
    • You may create subsections to group the changes, if you like
  • Use full sentences with correct case and punctuation
  • Refer to relevant Issue if applicable