fix class_like reparenting to namespaces.

• See tox -e py -- -k cpp_pimpl before this, somehow something changed and class_like were not getting reparented to namespaces. It is not clear how only this test was broken...

    Expected class hierarchy:
    --------------------------------------------
    - :ref:`namespace_pimpl`
        - :ref:`namespace_pimpl__detail`
            - :ref:`exhale_class_classpimpl_1_1detail_1_1EarthImpl`
            - :ref:`exhale_class_classpimpl_1_1detail_1_1JupiterImpl`
        - :ref:`exhale_class_classpimpl_1_1Earth`
        - :ref:`exhale_class_classpimpl_1_1Earth__v2`
        - :ref:`exhale_class_classpimpl_1_1EarthImpl`
        - :ref:`exhale_class_classpimpl_1_1Jupiter`
        - :ref:`exhale_class_classpimpl_1_1Jupiter__v2`
        - :ref:`exhale_class_classpimpl_1_1JupiterImpl`
        - :ref:`exhale_class_classpimpl_1_1Planet`
    Generated class hierarchy:
    --------------------------------------------
    - :ref:`namespace_pimpl`
        - :ref:`exhale_class_classpimpl_1_1Earth`
        - :ref:`exhale_class_classpimpl_1_1Earth__v2`
        - :ref:`exhale_class_classpimpl_1_1Jupiter`
        - :ref:`exhale_class_classpimpl_1_1Jupiter__v2`
        - :ref:`exhale_class_classpimpl_1_1Planet`
    - :ref:`exhale_class_classpimpl_1_1detail_1_1EarthImpl`
    - :ref:`exhale_class_classpimpl_1_1detail_1_1JupiterImpl`
    - :ref:`exhale_class_classpimpl_1_1EarthImpl`
    - :ref:`exhale_class_classpimpl_1_1JupiterImpl`
  

Fix warnings on CMP0079 (cxx tests, require CMake 3.13+).

Correct download of Catch2 file(DOWNLOAD [url]).

Remove AppVeyor and Travis builds.

Add GitHub Actions builds.

Fix linkcheck errors.

Figure out what's going on with the coverage changes.