Add folder for python customizations: __sitecustomize__

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-12-16.09:42:43.259>
labels = ['type-feature', 'library', '3.10']
title = 'Add folder for python customizations: __sitecustomize__'
updated_at = <Date 2021-05-24.20:31:53.995>
user = 'https://github.com/mariocj89'

bugs.python.org fields:

activity = <Date 2021-05-24.20:31:53.995>
actor = 'vstinner'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-12-16.09:42:43.259>
creator = 'mariocj89'
dependencies = []
files = []
hgrepos = []
issue_num = 42654
keywords = []
message_count = 5.0
messages = ['383129', '383138', '383139', '383141', '383164']
nosy_count = 5.0
nosy_names = ['jaraco', 'ncoghlan', 'steven.daprano', 'mariocj89', 'FFY00']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue42654'
versions = ['Python 3.10']