Unpin pyflakes version

[hakbailey]

What does this PR do?

Removes pyflakes from Pipfile, unpinning the version and letting it be handled as a Pylama subdependency.

Helpful background context

Pylama had a bug with the latest version of pyflakes, so we had to pin the pyflakes version to an older one in order for the linters to work. Pylama has released a version with a fix so we can unpin pyflakes now.

How can a reviewer manually see the effects of these changes?

Run make install and confirm that the installed pyflakes version is 2.5.0 and make lint works as expected (or just review the Github actions log for the same info).

Includes new or updated dependencies?

YES

Developer

• All new ENV is documented in README (or there is none)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes