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.

Author: sybrenstuvel

.travis.yml

@@ -8,6 +8,7 @@ cache: pip
 python:
   - "3.6"
   - "3.7"
+  - "3.8"
 
 matrix:
   include:

CHANGELOG.md

@@ -3,6 +3,7 @@
 
 ## Version 4.1 - in development
 
+- Added support for Python 3.8.
 - Dropped support for Python 2 and 3.4.
 - Added type annotations to the source code. This will make Python-RSA easier to use in
   your IDE, and allows better type checking.