activate.ps1 in venv for Windows should encoded with BOM

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-04-12.08:27:01.281>
labels = ['3.7', '3.8', 'OS-windows', 'type-crash']
title = 'activate.ps1 in venv for Windows should encoded with BOM'
updated_at = <Date 2019-04-12.18:06:53.148>
user = None

bugs.python.org fields:

activity = <Date 2019-04-12.18:06:53.148>
actor = 'steve.dower'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Windows']
creation = <Date 2019-04-12.08:27:01.281>
creator = '\xec\xa0\x95\xed\x95\x9c\xec\x86\x94'
dependencies = []
files = []
hgrepos = []
issue_num = 36609
keywords = []
message_count = 2.0
messages = ['340014', '340092']
nosy_count = 5.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'steve.dower', '\xec\xa0\x95\xed\x95\x9c\xec\x86\x94']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue36609'
versions = ['Python 3.7', 'Python 3.8']