fix default value for cursor.fetchmany() parameter

[jpz]

Add missing size = self.arraysize default value, as per spec and comments.

What do these changes do?

.fetchmany(size) should default the size parameter to cursor.arraysize if not supplied. The comments in the source code say as much but it is missing. This patch corrects this.

Are there changes in behavior for the user?

No changes, other than code that should have worked will now work as expected (rather than being syntactically incorrect.)

Related issue number

https://github.com/aio-libs/aioodbc/issues/377

Checklist

• [X ] I think the code is well written
• [ ] Unit tests for the changes exist
• [ X] Documentation reflects the changes - yes, no change required
• [ ] 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.