Formatting for entire openfl namespace (including experimental)

[MasterSkepticista]

Pylint score before any changes: 5.45/10
Edit: Pylint score after this PR: 8.09/10

• Convert all imports to be absolute.
• Migrate all imports to be on top-level.
• Sort all imports.
• Re-insert updated Copyright headers in all impacted files.
• Update Github actions workflow to use the new linting script.
• Format code.
• Clear all CI tests.
• Works with OpenFL-Security.

Reviewers, pls note:

openfl/tests/openfl/interface/test_aggregator_api.py

Lines 70 to 77 in ac3ed37

	# NOTE: This test is disabled because of cryptic behaviour on calling
	# _certify(). Previous version of _certify() had imports defined within
	# the function, which allowed theses tests to pass, whereas the goal of the
	# @mock.patch here seems to be to make them dummy. Usefulness of this test is
	# doubtful. Now that the imports are moved to the top level (a.k.a out of
	# _certify()) this test fails.
	# In addition, using dummy return types for read/write key/csr seems to
	# obviate the need for even testing _certify().

[MasterSkepticista]

This has been a tiring one. Request this be merged as soon as we can (ideally before any open feature PRs), so future PRs can make use of the updated code practices.

[mansishr]

LGTM! Great work!

[kta-intel]

LGTM! Thanks for the effort. Code looks much cleaner now. This will be nice going forward

[nammbash]

Nice technical debt PR!. LGTM

[psfoley]

Great work, @MasterSkepticista! Approved and ready for merge