Fix pool size limit check for unlimited pools

[d-ryzhykau]

What do these changes do?

Fix pool size limit check for unlimited connection pools. AS IS: acquiring a connection on a pool with minsize=0, maxsize=0 blocks forever. AS EXPECTED: acquiring a connection on a pool with minsize=0, maxsize=0 returns a created connection.

Are there changes in behavior for the user?

These changes will allow to create a pool with settings minsize=0, maxsize=0.

Related issue number

https://github.com/aio-libs/aiopg/issues/311 - even though the TypeError has been fixed, it is still impossible to use a pool with minsize=0, maxsize=0.

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [ ] Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
  • .feature: Signifying a new feature.
  • .bugfix: Signifying a bug fix.
  • .doc: Signifying a documentation improvement.
  • .removal: Signifying a deprecation or removal of public API.
  • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[Pliner]

Thanks