**Security & Infrastructure Update**. :shipit:
I was planning to update to python 3.9.x. Github dependabot found security issue in Pillow dependency; Also, when I updated binary compares of .pdf test files were failing; So I changed the comparison method until I find a better one.
* https://github.com/hasii2011/pyumldiagrams/issues/31 Update how to compare PDF files
* https://github.com/hasii2011/pyumldiagrams/issues/29 Update docs to match current release
* https://github.com/hasii2011/pyumldiagrams/issues/28 Update to fdp2 version 2.3.2
* https://github.com/hasii2011/pyumldiagrams/issues/24 Figure out how to support OpenGL in pygame 2.0.0
* https://github.com/hasii2011/pyumldiagrams/issues/21 Update to Python 3.9.0