[getpass] Implement a "strict" mode for getuser()

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-02-04.05:55:34.124>
labels = ['type-feature', 'library', 'OS-windows', '3.11']
title = 'Implement a "strict" mode for getpass.getuser()'
updated_at = <Date 2022-02-05.03:39:16.470>
user = 'https://github.com/eryksun'

bugs.python.org fields:

activity = <Date 2022-02-05.03:39:16.470>
actor = 'eryksun'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'Windows']
creation = <Date 2022-02-04.05:55:34.124>
creator = 'eryksun'
dependencies = []
files = []
hgrepos = []
issue_num = 46631
keywords = []
message_count = 2.0
messages = ['412495', '412556']
nosy_count = 5.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46631'
versions = ['Python 3.11']