finish CI move to GHA

[shimanek]

Just poking around trying to finish #1074 and maybe make progress toward #1117. With this it seems like GitHub actions based CI is in a working state; publishing workflow obviously untested and so far remains unchanged from the draft by @kernc. Relevant to recent discussion around #847. Is this viable? What's left?

Passing checks here: shimanek#4

[pep8speaks]

Hello @shimanek! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file benchmarks/bench_ml.py:

Line 155:80: E501 line too long (88 > 79 characters)
Line 247:80: E501 line too long (85 > 79 characters)
Line 249:80: E501 line too long (96 > 79 characters)
Line 286:80: E501 line too long (87 > 79 characters)
Line 287:80: E501 line too long (86 > 79 characters)

• In the file doc/conf.py:

Line 26:1: E402 module level import not at top of file
Line 27:1: E402 module level import not at top of file

• In the file examples/hyperparameter-optimization.py:

Line 45:1: E402 module level import not at top of file

• In the file examples/interruptible-optimization.py:

Line 52:1: E305 expected 2 blank lines after class or function definition, found 1
Line 52:80: E501 line too long (99 > 79 characters)

• In the file examples/plots/partial-dependence-plot-with-categorical.py:

Line 22:1: E402 module level import not at top of file

• In the file examples/plots/visualizing-results.py:

Line 113:1: E402 module level import not at top of file

• In the file examples/sampler/sampling_comparison.py:

Line 51:1: E402 module level import not at top of file

• In the file setup.py:

Line 13:80: E501 line too long (83 > 79 characters)

• In the file skopt/__init__.py:

Line 38:80: E501 line too long (81 > 79 characters)

• In the file skopt/learning/forest.py:

Line 197:80: E501 line too long (82 > 79 characters)
Line 393:80: E501 line too long (82 > 79 characters)

• In the file skopt/tests/test_space.py:

Line 97:80: E501 line too long (83 > 79 characters)

[xmatthias]

I don't think this is correct.
At least one of the above steps MUST build the package, otherwise the dist directory is empty.

something around the following should work ...

      - name: Build package
        if: env.TEST_WITH_COVERAGE == 'true'
        run: |
          pip install --upgrade build
          python -m build --sdist --wheel --outdir dist/