DX: Specify Click versions under test.

- Cherry picked from upstream:

    click-contrib/click-aliases.git 0352616 2019-08-06

      Add click 7 support

      Closes click-contrib#2

tox.ini

@@ -1,4 +1,9 @@
 [testenv]
+toxenv = py{35,36,37}-click{70,71,7x}
 passenv = LANG
-deps = pytest
+deps =
+  click70: click==7.0
+  click71: click==7.1
+  click7x: click>7,<8
+  pytest
 commands = py.test