tox.ini: don't run xenial env by default

It fails with more recent version of Python (at least 3.8), so it
doesn't make sense as a default environment for developers who will,
increasingly, be using a version of Python with which xenial's pytest is
incompatible.

(Note that this _does not_ remove the xenial env from our CI, so PRs
_will_ still be tested against the xenial set of libraries.)

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py3, xenial, pycodestyle, pyflakes, pylint
+envlist = py3, pycodestyle, pyflakes, pylint
 recreate = True
 
 [testenv]