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.
Makefile: Combine static and beauty targets into check.
Makefile: Refine and expand pylint testing.
Makefile: Perform check on each build.
Merge pull request #69 from Renelvon/flake8

Replace pep8 with flake8. Strengthen static analysis. Check code on each build.
AST, Parse, Parse test: Separate constants from functions.

* Add ConstantDef node.
* Parse parametre-less function definitions as constants
* Require param_list to be non-empty (i.e. param_seq)
* Adapt parsing unit tests accordingly.