tox.ini: introduce a xenial-dev testenv, executed by default

This environment is identical to [testenv:xenial], but uses the earliest
version of pytest that works with Python 3.8.

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py3, xenial, pycodestyle, pyflakes, pylint
+envlist = py3, xenial-dev, pycodestyle, pyflakes, pylint
 recreate = True
 
 [testenv]
@@ -55,11 +55,15 @@ commands =
     {envpython} -m sphinx {posargs:doc/rtd doc/rtd_html}
     doc8 doc/rtd
 
-[testenv:xenial]
-commands =
-  python ./tools/pipremove jsonschema
-  python -m pytest {posargs:tests/unittests cloudinit}
-basepython = python3
+[xenial-shared-deps]
+# The version of pytest in xenial doesn't work with Python 3.8, so we define
+# two xenial environments: [testenv:xenial] runs the tests with exactly the
+# version of pytest present in xenial, and is used in CI.  [testenv:xenial-dev]
+# runs the tests with the lowest version of pytest that works with Python 3.8,
+# 3.0.7, but keeps the other dependencies at xenial's level.
+#
+# (This section is not a testenv, it is used to maintain a single definition of
+# the dependencies shared between the two xenial testenvs.)
 deps =
     # requirements
     jinja2==2.8
@@ -76,10 +80,27 @@ deps =
     # test-requirements
     httpretty==0.9.6
     mock==1.3.0
-    pytest==2.8.7
     unittest2==1.1.0
     contextlib2==0.5.1
 
+[testenv:xenial]
+commands =
+  python ./tools/pipremove jsonschema
+  python -m pytest {posargs:tests/unittests cloudinit}
+basepython = python3
+deps =
+    # Refer to the comment in [xenial-shared-deps] for details
+    {[xenial-shared-deps]deps}
+    pytest==2.8.7
+
+[testenv:xenial-dev]
+commands = {[testenv:xenial]commands}
+basepython = {[testenv:xenial]basepython}
+deps =
+    # Refer to the comment in [xenial-shared-deps] for details
+    {[xenial-shared-deps]deps}
+    pytest==3.0.7
+
 [testenv:tip-pycodestyle]
 commands = {envpython} -m pycodestyle {posargs:cloudinit/ tests/ tools/}
 deps = pycodestyle