Add a lint-python section to Makefile which pip installs flake8 and then executes:

• flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

Based on the experimentation done at https://travis-ci.com/nodejs/node/builds/79706150 - #21942

These tests currently exclude several directories and files:

• ./deps/npm/node_modules/node-gyp -- this repo currently has no automated testing
• ./deps/v8 -- contains several Python 3 only features such as async def
• ./src/noperfctr_macros.py -- contains "macros" which Python AST views as syntax errors
• ./src/notrace_macros.py -- contains "macros" which Python AST views as syntax errors
• ./tools -- contains "macros" which Python AST views as syntax errors

Hopefully this list can be reduced in future PRs with code modifications or the use of # noqa

E901,E999,F821,F822,F823 are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety.

• F821: undefined name name
• F822: undefined name name in __all__
• F823: local variable name referenced before assignment
• E901: SyntaxError or IndentationError
• E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines