Maintenance + Python 3.8

• 竜 TatSu is now only tested against Python 3.8. Earlier versions of Python are now deprecated, and Python 2.X versions are no longer supported (@apalala).
• Apply nameguard only if token[0].isalpha(). This solves a regression afecting previous TatSu and Grako grammars (@apalala).
• Remove pygraphviz from develoment requirements, as it doesn't build under Py38 (@apalala)
• #56 Include missing tatsu/g2e/antlr.ebnf in distribution (@apalala)
• #138 Reimplement the calculation of FIRST, FOLLOW, and LOOKAHEAD sets using latest theories. For now, this should improve parser error reporting, but should eventually enable the simplification of parsing of leftrec grammars (@apalala).
• #153 Import ABCs from collections.abc (@tirkarthi)
• Remove support for Cythonand pypy3 (@apalala).