* Removed Python 2 comptibility code.

* Fixed bug with unused custom error callback.
* Added support for building with `XMLSEC_NO_XSLT` macro.
* Reactivated documentation builds on RTD.
* Switched docs theme to `furo`.

* Turned off building Python 2.7 wheels in CI pipelines

* Fixed `libxmlsec1` version checks

* 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 orig…

…inal setup

Various improvements related to packaging, distribution, CI and docum…

…entation. 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.

Fixed error when building the source dist on older platforms due to a…

…ccidental drop of escaping the ``XMLSEC_CRYPTO`` macro value.

Merge pull request xmlsec#87 from mehcode/develop

Fixed windows installation

Merge pull request xmlsec#87 from mehcode/develop

Fixed windows installation

Merge pull request xmlsec#76 from mintel/XMLSEC_CRYPTO-quotes

Fix build on CentOS 6 w/ xmlsec1-1.2.19-3

Fix installation via pip