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 #87 from mehcode/develop

Fixed windows installation

Merge pull request #87 from mehcode/develop

Fixed windows installation

Merge pull request #76 from mintel/XMLSEC_CRYPTO-quotes

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

Fix installation via pip

Merge pull request #69 from sindrig/master

Fix name collision with lxml's setupinfo.py

Merge pull request #68 from mehcode/windows_build

Introduced windows support

Merge pull request #67 from mehcode/develop

Merge branch develop to master (prepare version 1.2.0)