Remove excess Travis CI jobs

.travis.yml

@@ -1,24 +1,13 @@
 dist: xenial
 language: python
-cache:
+cache: pip
-  pip: true
-  directories:
-    - $HOME/.cache/pre-commit
 
 notifications:
   irc: "chat.freenode.net#pil"
 
-# Run fast lint first to get fast feedback.
-# Run slower CPUs next, to give them a headstart and reduce waiting time.
-# Then run the remainder.
-
 matrix:
   fast_finish: true
   include:
-    - python: "3.6"
-      name: "Lint"
-      env: LINT="true"
-
     - python: "3.6"
       arch: arm64
     - python: "3.7"
@@ -26,42 +15,15 @@ matrix:
     - python: "3.8"
       arch: s390x
 
-    - python: "pypy3.6-7.3.1"
-      name: "PyPy3 Xenial"
-    - python: "3.9"
-      name: "3.9 Xenial"
-      services: xvfb
-    - python: "3.8"
-      name: "3.8 Xenial"
-      services: xvfb
-    - python: '3.7'
-      name: "3.7 Xenial PYTHONOPTIMIZE=2"
-      env: PYTHONOPTIMIZE=2
-      services: xvfb
-    - python: '3.6'
-      name: "3.6 Xenial PYTHONOPTIMIZE=1"
-      env: PYTHONOPTIMIZE=1
-      services: xvfb
-
 install:
   - |
-    if [ "$LINT" == "true" ]; then
+    .ci/install.sh;
-      python3 -m pip install tox
-    else
-      .ci/install.sh;
-    fi
 
 script:
-- |
+  - |
-  if [ "$LINT" == "true" ]; then
-    tox -e lint
-  else
     .ci/build.sh
     .ci/test.sh
-  fi
 
 after_success:
-- |
+  - |
-  if [ "$LINT" == "" ]; then
     .ci/after_success.sh
-  fi