Fix CI, prepare test on Python 3.9 beta

[henryiii]

• Fixed broken Sphinx job in CI by pinning Sphinx < 3. We can possibly fix this later on the latest Breathe + Sphinx.
• Enabled NumPy on the 3.8 test
• Added new 3.9 Beta test, expected fail for now.
• Dropped the old PyPy 5.8 test because it was broken, added two PyPy 7.3 tests (2.7 and 3.6); these do (expected) fail, but we should be able to start fixing the tests, I hope. The 2.7 mode looks close (3-4 test failures), the 3.6 mode fails to build.

[henryiii]

This should have fixed Doxygen 3 + latest Breathe, I think, but did not:

breathe_doxygen_config_options = {
    'ENABLE_PREPROCESSING'   : 'YES',
    'MACRO_EXPANSION'        : 'YES',
}

[henryiii]

Okay, I think this is ready for review.

EDIT: Python 3.9 test is running on the built-in Python 3.6. Oops! Will fix.

EDIT: Fixed. Now it's hitting the warning and failing, as it should.

[rwgk]

Looks good to me.

[wjakob]

This looks fantastic! Thank you very much @henryiii. I'm also glad that you updated PyPy. The 5.x release was super-flaky and sometimes caused test failures, maybe the new version will address this.