chore: make ListMixin._list_filters always present

chore: make RESTObject._short_print_attrs always present

It's mostly an internal thing anyway and can be removed in 3.0.0

Always create ListMixin._list_filters attribute with a default value
of tuple().

This way we don't need to use hasattr() and we will know the type of
the attribute.

Always create RESTObject._short_print_attrs with a default value of
None.

This way we don't need to use hasattr() and we will know the type of
the attribute.

fix: correct ProjectFile.decode() documentation

Fix all issues reported by running: tox -e pep8 and enable pep8 as a linter check

ProjectFile.decode() returns 'bytes' and not 'str'.

Update the method's doc-string and add a type-hint.

ProjectFile.decode() returns the result of a call to
base64.b64decode()

The docs for that function state it returns 'bytes':
https://docs.python.org/3/library/base64.html#base64.b64decode

Fixes: #1403

While adding type-hints mypy flagged this as an issue. The third
argument to register_custom_action is supposed to be a tuple. It was
being passed as a string rather than a tuple of strings.

Local variable name is assigned to but never used

https://www.flake8rules.com/rules/F841.html