Xhtml2pdf

**🎉 New**

* Add encryption and password protection
* New WaterMark management system with new options
* Add Graphic builder
* Add signing pdfs (simple and pades)


**🐛 Bug-Fixes**

* Remove import cycle between utils and default
* Fixed link_callback construction of path
* Fixed path when is relative to current path

**⚠️ Deprecation**

* `xhtml` in pisa.CreatePDF support will removed on next release
* `XML2PDF` and `XHTML2PDF` will be removed on next release use `HTML2PDF` instead

**📘 Documentation**

* Add render pdf on documentation and add some html example.
* Include graphics examples


| Thanks to the following people on GitHub for contributing to this release:
| *marcelagz* for graphics support :)

* Drop python 2 support.
* Remove most of python 2 code and cleanup
* Update packages dependencies
* Remove six dependency and update Readme
* Set timeout in https options
* Add new file manager approach using factory method, now new classes deal with different types of data
B64InlineURI, LocalProtocolURI, NetworkFileUri, LocalFileURI, BytesFileUri
* Now getColor return None when None is passed ignoring default value, but return default if bool(data) == false
* rtl languages reversed lines added as a ParaFrag (note: not fully supported yet)
* Check if Paragraph has 'rtl' attribute (note: not fully supported yet)
* Fix UnboundLocalError in reportlab_paragraph (585) (586)
* Remove usage of getStringIO (590) removed form reportlab
* Change test for github workflow using only Linux
* Add Python 3.9, 3.10
* Switch from PyPDF2 to PyPDF3
* Add SVG support
* Update package information.
* Allow call tests using make.

Update link_callback documentation.
Stylize code lines in documentation.
Fixed cgi escape util on setup version.
Add test to python 3.7 and 3.8.
Fixed width assignation on fragments.
Support urllib in python 3 and python 2.
Add em unit support.
Repair base64 unscaped string.
Fixed urlparse when urls has parameters.
Fixed i_rgbcolor support.

Include new python version in test and change requirements to force html5lib to 1.0.1.

-----------------

* Improve python3 support - thanks ***luisza, andreyfedoseev and flupzor ***
* Include new Httplibs options - thanks ***luisza***
* Support to background image - thanks ***flupzor***
* Remove python23 support - thanks ***flupzor***
* Transparent images work again in Python 3 - thanks ***flupzor***
* Readthedocs integration - thanks ***luisza***
* Update Django demo site - thanks ***luisza***
* PEP8 and cleanup code - thanks ***luisza***
* Drop the turbogears module - thanks ***browniebroke***