Allow customizing python interpreter in venv.EnvBuilder

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-08-13.08:31:20.557>
labels = ['type-feature', 'library', '3.10']
title = 'Allow customizing python interpreter in venv.EnvBuilder'
updated_at = <Date 2020-08-13.11:35:17.299>
user = 'https://github.com/abbec'

bugs.python.org fields:

activity = <Date 2020-08-13.11:35:17.299>
actor = 'abbec'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-08-13.08:31:20.557>
creator = 'abbec'
dependencies = []
files = []
hgrepos = []
issue_num = 41538
keywords = ['patch']
message_count = 1.0
messages = ['375293']
nosy_count = 2.0
nosy_names = ['vinay.sajip', 'abbec']
pr_nums = ['21854']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41538'
versions = ['Python 3.10']