_enquote function uses 'unicode-internal', which was removed in Python 3.8.

[slotnick]

Describe the bug
While running under Python 3.8.1:
File "...lib/python3.8/site-packages/crossplane/builder.py", line 69, in _enquote
arg = codecs.decode(arg, 'unicode-internal')
LookupError: unknown encoding: unicode-internal

[framer777]

#76 merged