fix Cursor.mogrify type hint: it returns bytes, not str

[gjcarneiro]

What do these changes do?

the type hint for Cusor.mogrify is wrong. It returns bytes, not str (for proof, look for the unit test test_mogrify). This fixes the type hint, and inline doc.

Are there changes in behavior for the user?

Related issue number

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.

[codecov[bot]]

Codecov Report

Merging #879 (282b45c) into master (e131396) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #879   +/-   ##
=======================================
  Coverage   93.32%   93.32%           
=======================================
  Files          12       12           
  Lines        1574     1574           
  Branches      187      187           
=======================================
  Hits         1469     1469           
  Misses         73       73           
  Partials       32       32           

Impacted Files	Coverage Δ
aiopg/connection.py	95.66% <100.00%> (ø)

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[Pliner]

Thanks