Make WSGIRequestHandler easier to be customized by the user

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-05-20.15:54:01.331>
labels = ['3.7', '3.8', 'type-feature', 'library', '3.9']
title = 'Make WSGIRequestHandler easier to be customized by the user'
updated_at = <Date 2020-05-28.15:09:56.874>
user = 'https://github.com/Zheaoli'

bugs.python.org fields:

activity = <Date 2020-05-28.15:09:56.874>
actor = 'Manjusaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-05-20.15:54:01.331>
creator = 'Manjusaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40700
keywords = ['patch']
message_count = 2.0
messages = ['369465', '370224']
nosy_count = 1.0
nosy_names = ['Manjusaka']
pr_nums = ['20262']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue40700'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']