warnings.py should not raise AssertionError when validating arguments

[remilapeyre]

BPO	39912
Nosy	@remilapeyre
PRs	bpo-39912: Raise appropriate exceptions in filterwarnings() and simplefilter() #18878

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-03-09.14:05:44.963>
labels = ['library', '3.9']
title = 'warnings.py should not raise AssertionError when validating arguments'
updated_at = <Date 2020-03-09.14:15:03.051>
user = 'https://github.com/remilapeyre'

bugs.python.org fields:

activity = <Date 2020-03-09.14:15:03.051>
actor = 'remi.lapeyre'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-03-09.14:05:44.963>
creator = 'remi.lapeyre'
dependencies = []
files = []
hgrepos = []
issue_num = 39912
keywords = ['patch']
message_count = 1.0
messages = ['363732']
nosy_count = 1.0
nosy_names = ['remi.lapeyre']
pr_nums = ['18878']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue39912'
versions = ['Python 3.9']

[remilapeyre]

Currently simplefilter() and filterwarnings() don't validate their arguments when using -O and AssertionError is not the most appropriate exception to raise.

I will post a PR shortly.