Python 3.8 for tox (#6341)

... and update INSTALL.md to include py3.8.

We'll also have to update the buildkite pipeline to run it

INSTALL.md

@@ -36,7 +36,7 @@ that your email address is probably `user@example.com` rather than
 System requirements:
 
 - POSIX-compliant system (tested on Linux & OS X)
-- Python 3.5, 3.6, or 3.7
+- Python 3.5, 3.6, 3.7 or 3.8.
 - At least 1GB of free RAM if you want to join large public rooms like #matrix:matrix.org
 
 Synapse is written in Python but some of the libraries it uses are written in

changelog.d/6341.misc

+Add continuous integration for python 3.8.
\ No newline at end of file

tox.ini

 [tox]
-envlist = packaging, py35, py36, py37, check_codestyle, check_isort
+envlist = packaging, py35, py36, py37, py38, check_codestyle, check_isort
 
 [base]
 basepython = python3.7