Add an option to point to an additional "site" directory

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-09-26.17:59:03.754>
labels = ['type-feature', 'library']
title = 'Add an option to point to an additional "site" directory'
updated_at = <Date 2019-09-27.06:19:13.452>
user = 'https://bugs.python.org/MichaFronczyk'

bugs.python.org fields:

activity = <Date 2019-09-27.06:19:13.452>
actor = 'Micha\xc5\x82 Fronczyk'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-09-26.17:59:03.754>
creator = 'Micha\xc5\x82 Fronczyk'
dependencies = []
files = []
hgrepos = []
issue_num = 38289
keywords = []
message_count = 2.0
messages = ['353332', '353347']
nosy_count = 2.0
nosy_names = ['barry', 'Micha\xc5\x82 Fronczyk']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38289'
versions = []