Mutable module level variables

Mutable module level variables might lead to bugs

file: a.py

SOME_CONSTANT = [1,5,7]

file: b.py

from a import SOME_CONSTANT

if bla:
     SOME_CONSTANT.remove(5)

assert i in SOME_CONSTANT:

The correct thing to do would be to use:
a.py

SOME_CONSTANT = (1,5,7)

Mutable module level variables should be reported similar to B006: Do not use mutable data structures for argument defaults.

While this might lead to false positives we could either:

• Only signal on YELLING_CASE variables
• Make it an opinionated and thus disabled by default lint (in which case I would argue #57 belongs there as well)

Start by locating the existing B006 implementation and the plugin's check registration and test entry points; the issue provides a.py and b.py examples to use as the initial cases. Decide and document the handling of false positives, including YELLING_CASE variables or an opt-in check, then add coverage showing that mutable module-level variables are reported as intended.