Travis config update. (#22)

* test on supported Python 3 versions.
* switch away from test.sh for CI
* drop 2.7.  it's EOL.
diff --git a/.travis.yml b/.travis.yml
index 55e664f..c9383cc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,17 @@
 language: python
+python:
+  - "3.6"
+  - "3.7"
+  - "3.8"
+  - "3.9"
 os: linux
 arch:
   - amd64
   - ppc64le
 dist: focal
+install:
+  - pip install mock
+  - pip install --upgrade .
 script:
-  - ./test.sh
+  - python src/tests/portpicker_test.py
+  - PYTHONPATH=src python src/tests/portserver_test.py
diff --git a/test.sh b/test.sh
index ab53aa0..cbd0c69 100755
--- a/test.sh
+++ b/test.sh
@@ -1,25 +1,27 @@
 #!/bin/sh -ex
 
-echo 'TESTING under Python 3'
-mkdir -p build/test_envs/python3
-python3 -m venv build/test_envs/python3
-# Without --upgrade pip won't copy local changes over to a new test install
-# unless you've updated the package version number.
-build/test_envs/python3/bin/pip install --upgrade .
-build/test_envs/python3/bin/python3 src/tests/portpicker_test.py
+if which python3 >/dev/null ; then
+  echo 'TESTING under Python 3'
+  mkdir -p build/test_envs/python3
+  python3 -m venv build/test_envs/python3
+  # Without --upgrade pip won't copy local changes over to a new test install
+  # unless you've updated the package version number.
+  build/test_envs/python3/bin/pip install --upgrade pip
+  build/test_envs/python3/bin/pip install --upgrade .
+  build/test_envs/python3/bin/python3 src/tests/portpicker_test.py
 
-echo 'TESTING the portserver'
-PYTHONPATH=src build/test_envs/python3/bin/python3 src/tests/portserver_test.py
+  echo 'TESTING the portserver'
+  PYTHONPATH=src build/test_envs/python3/bin/python3 src/tests/portserver_test.py
+fi
 
 if which python2.7 >/dev/null ; then
-echo 'TESTING under Python 2.7'
-mkdir -p build/test_envs/python2
+  echo 'TESTING under Python 2.7'
+  mkdir -p build/test_envs/python2
   virtualenv --python=python2.7 build/test_envs/python2
   build/test_envs/python2/bin/pip install mock
+  build/test_envs/python2/bin/pip install --upgrade pip
   build/test_envs/python2/bin/pip install --upgrade .
   build/test_envs/python2/bin/python2 src/tests/portpicker_test.py
-else
-  echo 'Python 2.7 not found, not testing on that obsolete thing...'
 fi
 
 echo PASS