Fix incorrect Python version in CI

[nikfilippas]

Some years ago, when the Python package handling in CI was changed to conda, the Python version was still left as a specifier in the matrix. However, this doesn't do anything at all. Conda would just use Python 3.11 (the latest version) even though we had 3.8 specified.

Proof (build from master):

  Package                 Version  Build                Channel                    Size
─────────────────────────────────────────────────────────────────────────────────────────────────
  Install:
─────────────────────────────────────────────────────────────────────────────────────────────────
      ...
  + python                 3.11.0  he550d4f_1_cpython   conda-forge/linux-64       31MB
      ...

This PR moves the Python version inside of the Anaconda environment, so that we use the correct one.

[coveralls]

Pull Request Test Coverage Report for Build 4458802508

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.857%

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Totals
Change from base Build 4437584140:	0.0%
Covered Lines:	5480
Relevant Lines:	5600

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💛 - Coveralls

[damonge]

Also, what's up with the waiting tests?

[nikfilippas]

@damonge they have been replaced because the matrix has changed. We don't test (ubuntu-latest, py38) and (macos-latest, py38) any more. Just ubuntu and macos.