Update flake8 per-file ignores

[dstansby]

PR Summary

Update this to:

• Re-arrange some of the files into alphabetical order
• Remove some error ignores that we don't need any more

For reference I managed to 95% automate this by parsing the outptut of flake8 into a list of ingore rules:

from collections import defaultdict

with open('flake8.txt') as f:
    lines = f.readlines()

lines = [l.split(': ') for l in lines]
files = [l[0].split(':')[0] for l in lines]
codes = [l[1].split(' ')[0] for l in lines]

errs = defaultdict(set)
for f, c in zip(files, codes):
    errs[f].add(c)

errs = {k: list(v) for k, v in errs.items()}
for f in sorted(errs):

    err_str = ', '.join(sorted(errs[f]))
    print(f"{f}: {err_str}")

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst