Remove the Python 2 support

It should mostly still work (except for :Pyimport), but it won't pass all the tests anymore.
Since I'm dropping Python 2 in Jedi anyways, it makes sense to also remove it here.

.travis.yml

@@ -2,16 +2,9 @@ dist: bionic
 language: python
 python: 3.8
 env:
-  matrix:
+  - ENV=test
-    - ENV=test
+  - ENV=check
-    - ENV=check
+  - ENV=test_coverage
-    - ENV=test_coverage
-matrix:
-  include:
-    - env: ENV=test_coverage
-      python: 2.7
-    - env: ENV=test_coverage
-      python: 3.8
 install:
   - |
     if [ "$ENV" = "test" ]; then