Send Code Climate test report only for Python 3.8

[pv8]

Send Code Climate test report only when running tests on Python 3.8.

Because GitHub actions doesn't set GIT_BRANCH nor GIT_COMMIT_SHA, some workarounds had to be implemented:

• For actions/checkout@v2 or higher, GITHUB_HEAD_REF needs to be fetched:

  git fetch --no-tags --prune --depth=1 origin +refs/heads/$GITHUB_HEAD_REF:refs/remotes/origin/$GITHUB_HEAD_REF

• Set GIT_BRANCH and GIT_COMMIT_SHA before executing ./cc-test-reporter:

  GIT_BRANCH=$GITHUB_HEAD_REF GIT_COMMIT_SHA=$(git rev-parse origin/$GITHUB_HEAD_REF) ./cc-test-reporter ...