Warn about invalid PYTHONUSERBASE

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-04-06.12:59:15.393>
labels = ['interpreter-core', 'type-feature', '3.9']
title = 'Warn about invalid PYTHONUSERBASE'
updated_at = <Date 2020-04-11.11:54:27.408>
user = 'https://github.com/Volker-Weissmann'

bugs.python.org fields:

activity = <Date 2020-04-11.11:54:27.408>
actor = 'Volker Wei\xc3\x9fmann'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2020-04-06.12:59:15.393>
creator = 'Volker Wei\xc3\x9fmann'
dependencies = []
files = []
hgrepos = []
issue_num = 40203
keywords = []
message_count = 4.0
messages = ['365851', '365857', '366156', '366203']
nosy_count = 2.0
nosy_names = ['terry.reedy', 'Volker Wei\xc3\x9fmann']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40203'
versions = ['Python 3.9']