Remove BlockingPortal.__new__ hack

[gschaffner]

Changes

Removes the BlockingPortal.__new__ hack. See #256 (comment):

This change would likely allow us to remove the __new__() hack from BlockingPortal.

Also documents :raises NoEventLoopError: for BlockingPortal().__init__.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new
  features
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

[agronholm]

Looks great! Thanks!