tests: Use python-xdist for faster tests

Add mutmut for mutation testing ...

I haven't added a target for this yet but it's worth just adding it.

Example invocation: mutmut run --paths-to-mutate src/tokenize.py

tests: Add SPDX license headers

src: tokenize: Add some function comments

src: tokenize: Always append current buffer ...

Current will always have data in its buffer, so append it.

Bug found by mutmut.

tests: Add pytest-randomly

src: tokenizer: Add beginning of new tokenizer

make: Run tidy first to avoid flake8 errors

Change license to LGPL-2.1-only

tests: parse: Remove unused integers include ...

Found by flake8.

make: Add 'check' target ...

This is intended to be run before comitting changes.

tests: lexer: Drop error testing code ...

This code is awful:

- It reaches across multiple layers of abstractions
- It does a lot of string splitting and joining to rebuild code
- It only tests very specific cases because the main code is broken

Just dump it for now and start over.

tests: lexer: Add test for inserting an EndText in to test ...

This is BROKEN and fundamentally flawed, but I'm keeping it here for
reference in Git history.

tests: Clarify purpose of draw_error_endtext_remove

tests: Rename EndText test to EndText stray test

tests: parse: Use repr() to print string values

src: parse: Fix uppercase repr value

tests: lexer: Test that removing an EndText causes an error ...

This only applies to the last Text element in a soup.

parse: Use __str__ for ParseError printing ...

Exceptions don't use __repr__ but __str__ for printing

tests: lexer: Check error location

tests: lexer: Test stray EndText errors are caught

tests: Clarify draw_world_salad

tests: parser: Remove unused includes ...

Found by flake8.

src/tests: Fix conditional statements ...

Don't directly compare with ==, use 'not' or 'is'.

Found by flake8.

src: parse: Don't assign unused variables ...

Found by flake8.

src: interp: Fix copy paste error in Equals ...

Found by flake8.

tests: lexer: Include shebang in valid tokens ...

Not sure how I missed this. Bug found by flake8.

tests: lexer: Include token type in equality comparison ...

Oops! Found this bug with mutation testing, not by review.

lint: Add flake8 command

tests: lexer: Split up soup functions and valid token testing ...

This will help when we add error injection