Add support for Python 3.8

Supporting Python 3.8 not only required configuring Tox and Travis to
run the tests on that version, but also required updating the
dependencies. Without that update, `pluggy` wouldn't work properly.

1 files changed, 1 insertions, 1 deletions

diff --git a/tox.ini b/tox.ini
index 3cca8dc..0446ef5 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 # Environment changes have to be manually synced with '.travis.yml'.
-envlist = py35,py36,p37
+envlist = py35,py36,p37,p38
 isolated_build = true
 
 [pytest]