Xmlsec

* XMLSec 1.3.x compliance
* Make DES/3DES/KW-3DES support conditional on DES availability in XMLSec
* PKCS11 support
* Improved libxml2 version check
* Several test improvements

**Full Changelog**: https://github.com/xmlsec/python-xmlsec/compare/1.3.13...1.3.14

* XMLSec 1.3.* compliance
* Make DES/3DES/KW-3DES support conditional on DES availability in XMLSec
* PKCS11 support
* Improved libxml2 version check
* Several test improvements

**Note**: Not compatible with XMLSec 1.3.3, use https://github.com/lsh123/xmlsec/releases/tag/1.3.4-rc1
**Full Changelog**: https://github.com/xmlsec/python-xmlsec/compare/1.3.13...1.3.14-rc1

* Add CI for Python 3.10, `manylinux2` SOABI, `musllinux1` SOABI and OpenSUSE Tumbleweed
* Introduce linting with `pre-commit`
* Rewrite PEP 484 stubs
* xmlsec workaround for ghmehcode/python-xmlsec84 - Stanislav Levin (mcepl)
* Resolve key loading issue on big-endian systems. - Chris MacNaughton (ChrisMacNaughton)
* Fix building wheel when using `PYXMLSEC_STATIC_DEPS=true`
* setup.py: Fix typo in PYXMLSEC_LIBXSLT_VERSION - AbdealiJK (AbdealiJK)
* Switch on modern headers lxml - Alexander Shadchin (shadchin)
* use build to build wheels - Thomas Grainger (graingert)
* remove pkgconfig as it's only a build-system requirement - Thomas Grainger (graingert)
* exclude lxml 4.7 because the wheels are missing the lxml includes - Thomas Grainger (graingert)

* Added support for registering custom xmlsec IO callbacks
* Added support for building without MD5 transforms
* Added support for PEP 539 for Python 3.7 and newer
* Using `lxml-stubs` package instead of custom LXML stubs

* Removed Python 2 compatibility 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