Requirements: Replace pep8 with flake8.

flake8 runs all checks of pep8 and then some more. Hooray!
Makefile: Unleash flake8.
Makefile: Refine and expand flake8 testing.
Main: Fix flake8 warning.
Lex test: Fix flake8 warnings.
Parse test: Fix flake8 warnings.
Type test: Fix flake8 warnings.