Improve ci.yml

[sobolevn]

Changes:

1. Updates codecov version to the latest one
2. Fixes cache key, it should include both txt files ('pyproject.toml' does not exist) and python version

[codecov]

Codecov Report

Merging #407 (de8ab61) into master (bbc8f7e) will not change coverage.
The diff coverage is n/a.

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[hugovk]

We can actually remove actions/cache as a separate run and use the built-in pip caching form actions/setup-python:

https://github.com/actions/setup-python#caching-packages-dependencies

Something like this:

- uses: actions/setup-python@v3
  with:
    python-version: ${{ matrix.python-version }}
    cache: 'pip'
    cache-dependency-path: |
      dev-requirements.txt
      requirements.txt

---

Edit: this stuff included in #416:

This caching of course only applies to actions/setup-python, and not deadsnakes/action used for the *-dev versions. But:

1. we're not actually testing any -dev versions

2. if we do want to test -dev, actions/setup-python now also supplies them. One important difference: actions/setup-python supplies the latest alpha/beta/RC, whereas deadsnakes/action gives a nightly build. Usually the latest alpha/beta/RC is enough? If so we can remove:

      - uses: deadsnakes/[email protected]
        if: endsWith(matrix.python-version, '-dev')
        with:
          python-version: ${{ matrix.python-version }}

---

Edit: this stuff included in #416:

Finally, we can update most (all?) of GitHub's own actions/* from v2 to v3.