Disable all but 2.7 and nightly testing.

I have to keep pestering travis to refill the credits I need for
testing.

Since github is testing the 3.x stuff, I only need to use travis for
latest devel (nigthly) and 2.7 python. 2.7 support has been dropped
from github.

This will hopefully reduce the number of times that I look to travis
for results and find that it didn't run because of exhausted cedits.

Author: rouilj

.travis.yml

@@ -22,11 +22,11 @@ dist: focal
 
 python:
   - 2.7
-  - 3.10.4
+#  - 3.10.4
 #  - 3.9
 #  - 3.8
-  - 3.6
-  - 3.11-dev
+#  - 3.6
+#  - 3.11-dev
   - nightly
 #  - pypy3