add black, flake8, reorder imports to pre-commit

[spyoungtech]

• Applies black formatting to FreeSimpleGUI library code and pre-commit config
• adds flake8 to pre-commit configuration
• fixes over 300 (non-formatting) flake8 violations in the main FreeSimpleGUI library
• Removes a lot of dead code
• Removes unneeded compat code targeting Python 3.4 (code was already syntax-incompatible with Python<3.6 anyhow)
• applies pyupgrade targeting 3.6