Xmlsec

* More tests added to increase test coverage
* Various small fixes for segfaults found
* Added `xmlsec.base64_default_line_size()` function to set the default maximum columns size for base64 encoding
* Added AES-GCM transform constants
* Added Python 3.9 support
* Recording code coverage also in MacOS jobs

Escape the XMLSEC_CRYPTO macro value, just as it was done in the original setup

Various improvements related to packaging, distribution, CI and documentation. Notable changes:

* Added [PEP 484](https://www.python.org/dev/peps/pep-0484/) type stubs.
* Moved documentation to RTD.
* Fixed segfault when trying to access missing attributes of some constants.
* Fixed building error with Python 2.7 on Windows.
* Added CI pipelines for ``manylinux2010`` reference system and MacOS.
* Reworked setup script, preparing for [PEP 571](https://www.python.org/dev/peps/pep-0571/) wheels.
* Prohibited installation for unsupported Python versions.
* Using ``setuptools-scm`` for dist version calculation from Git tags.

- no changes in comparison 1.3.5

- fixed installation issues for all platforms

- minor installation issue fix