Enforce max line length of 79 characters as in PEP-8

The majority of changed code in pmbootstrap is done with the max line length of 79 in mind already, as it is recommended by PEP-8:

In other repositories like builds.postmarketos.org this line length is enforced already. It would be good to have it enforced in pmbootstrap as well.

TODO:

Remove E501 from _ignores and remove the comment here
Run the script, and adjust all files that have a too long line length
(When changing lines that have string concatenation ("hello " + somevar + " world"), feel free to refactor them to f-strings (f"hello {somevar} world") while at it. I'm slowly converting all string concatenations in pmbootstrap to f-strings when touching related code.)
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