This updates a few pieces of code to fix up compatibility with Python

3.9. This consisted of a deprecated method use

(HTMLParser.unescape()) and some code that passed a byte string into

urlopen().
Since Python 3 support is now official, the package-level checks to

enable it only if the $RB_PY3_HAS_NO_OFFICIAL_SUPPORT variable is set

have been removed, and package metadata has been updated appropriately.

Unit tests pass for all versions of Python.
Verified that I could build packages on Python 2 and 3.