swift-package-manager

[Python] Update linter rules

In https://github.com/apple/swift/commit/d5326bfdc486d95b572446378a13fda6d9626b2c,
@practicalswift updated the Swift project to no longer ignore
individual linter rules globally. Update the SwiftPM project to do the
same.

In addition, fix all remaining linter rule violations. Most of these
involved splitting up lines. One spot involved using `endswith()` with
a tuple argument to check against multiple suffixes at once.

To lint the Python code in the project:

    $ flake8

To install flake8:

    $ pip install flake8

See https://flake8.readthedocs.org/en/latest/ for details.