pvlib codebase not compliant with flake8

[ramaroesilva]

Describe the bug
As mentioned by @echedey-ls in #2637, since the GitHub CI only covers the diffs within a PR, the codebase remains unchecked.

Expected behavior
Be flake8-compliant, although some nuances may raise exceptions (or existing exceptions may also be revised).

Versions:

• pvlib.__version__: 0.13.1

Additional context
Will address this in two separate PRs to make it easier to review (one today, another some other time).