Deprecate and eventually remove macOS-only PYTHONEXECUTABLE environ variable

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-03-02.17:45:33.785>
labels = ['OS-mac', '3.11', 'docs']
title = 'Deprecate and eventually remove macOS-only PYTHONEXECUTABLE environ variable'
updated_at = <Date 2022-03-02.17:45:33.785>
user = 'https://github.com/ned-deily'

bugs.python.org fields:

activity = <Date 2022-03-02.17:45:33.785>
actor = 'ned.deily'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'macOS']
creation = <Date 2022-03-02.17:45:33.785>
creator = 'ned.deily'
dependencies = []
files = []
hgrepos = []
issue_num = 46901
keywords = []
message_count = 1.0
messages = ['414382']
nosy_count = 4.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'docs@python', 'steve.dower']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue46901'
versions = ['Python 3.11']