Run py38 under coverage too

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = typing,lint,py27,py34,py35,py36,py37,pypy,pypy3,manifest,docs,pypi-description,changelog,coverage-report
+envlist = typing,lint,py27,py34,py35,py36,py37,py38,pypy,pypy3,manifest,docs,pypi-description,changelog,coverage-report
 isolated_build = True
 
 
@@ -27,6 +27,16 @@ extras = {env:TOX_AP_TEST_EXTRAS:tests}
 commands = coverage run --parallel -m pytest {posargs}
 
 
+[testenv:py38]
+# Python 3.6+ has a number of compile-time warnings on invalid string escapes.
+# PYTHONWARNINGS=d and --no-compile below make them visible during the Tox run.
+install_command = pip install --no-compile {opts} {packages}
+setenv =
+    PYTHONWARNINGS=d
+extras = {env:TOX_AP_TEST_EXTRAS:tests}
+commands = coverage run --parallel -m pytest {posargs}
+
+
 [testenv:coverage-report]
 basepython = python3.7
 skip_install = true