Add separate test environment for pytz

The separate tests with and without pytz are done to try to minimize the
possibility of implicit dependencies on pytz.

.travis.yml

@@ -178,7 +178,8 @@ install:
         $PYTEST_PEP8 \
         pytest-rerunfailures \
         pytest-timeout \
-        pytest-xdist
+        pytest-xdist \
+        tox
   - |
     # Install matplotlib
     python -mpip install -ve .
@@ -195,6 +196,7 @@ before_script: |
 script: |
   echo "Calling pytest with the following arguments: $PYTEST_ADDOPTS"
   python -mpytest
+  tox -e pytz
 
 before_cache: |
   rm -rf $HOME/.cache/matplotlib/tex.cache

tox.ini

@@ -14,3 +14,11 @@ commands =
 deps =
     numpy
     pytest
+
+[testenv:pytz]
+changedir = /tmp
+commands =
+    pytest -m pytz {toxinidir}
+deps =
+    pytest
+    pytz