Exclude unsupported combinations from build matrix

.travis.yml

@@ -12,6 +12,23 @@ env:
   - DJANGO=Django==1.6
   - DJANGO=Django==1.7
   - DJANGO=Django
+exclude:
+  ### These older Django version don't support Python 3.5:
+  # AttributeError: module 'html.parser' has no attribute 'HTMLParseError'
+  - python: 3.5
+    env: DJANGO=Django==1.5
+  - python: 3.5
+    env: DJANGO=Django==1.6
+  - python: 3.5
+    env: DJANGO=Django==1.7
+
+  ### Too old Python for the newest Django
+  # ImportError: cannot import name find_spec
+  - python: 3.3
+    env: DJANGO=Django
+  - python: pypy3  # Should be removed once pypy catches up with Python 3.4
+    env: DJANGO=Django
+
 script:
   - pip install $DJANGO coverage
   - pip install .