Add inspect.Signature.from_text().

GitHub fields:

assignee = 'https://github.com/ericsnowcurrently'
closed_at = None
created_at = <Date 2019-07-04.00:11:26.728>
labels = ['type-feature', 'library', '3.9']
title = 'Add inspect.Signature.from_text().'
updated_at = <Date 2020-03-06.20:46:18.865>
user = 'https://github.com/ericsnowcurrently'

bugs.python.org fields:

activity = <Date 2020-03-06.20:46:18.865>
actor = 'yselivanov'
assignee = 'eric.snow'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-07-04.00:11:26.728>
creator = 'eric.snow'
dependencies = []
files = []
hgrepos = []
issue_num = 37497
keywords = []
message_count = 5.0
messages = ['347250', '347251', '362765', '363535', '363551']
nosy_count = 2.0
nosy_names = ['eric.snow', 'yselivanov']
pr_nums = ['14579']
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue37497'
versions = ['Python 3.9']