Update PostgreSQL versions used in tests

[and-semakin]

What do these changes do?

Tests were using outdated/deprecated PostgreSQL versions. For better results tests should run over an actual PostgreSQL version.

I also had to remove some assertions in tests since they decided to phase out oids feature since PostgreSQL 12. See: https://stackoverflow.com/a/57022291/10650942

Are there changes in behavior for the user?

No. Only in tests.

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 #865 (1655124) into master (6df818c) will decrease coverage by 0.06%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #865      +/-   ##
==========================================
- Coverage   93.43%   93.36%   -0.07%     
==========================================
  Files          11       11              
  Lines        1568     1568              
  Branches      186      186              
==========================================
- Hits         1465     1464       -1     
- Misses         72       73       +1     
  Partials       31       31              

Impacted Files	Coverage Δ
aiopg/connection.py	95.66% <0.00%> (-0.18%)	:arrow_down:

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[and-semakin]

The PR template is a little bit outdated. It mentions some CHANGES folder which I don't see anymore.

[Pliner]

Thanks for the contribution.