Update Travis to build with Python 3.10 and add 3.10 to the classifiers, and drop 3.7 and 3.8 whose builds are failing

Fixes #83.

Author: JoshData

.travis.yml

@@ -1,14 +1,14 @@
 os: linux
-dist: xenial
+dist: bionic
 language: python
 cache: pip
 
 python:
-#- '2.7'
 - '3.6'
-- '3.7'
-- '3.8'
+#- '3.7'
+#- '3.8'
 - '3.9'
+- '3.10'
 
 install:
 - make install

setup.cfg

@@ -19,6 +19,7 @@ classifiers =
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
     Topic :: Software Development :: Libraries :: Python Modules
 keywords = email address validator