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

@@ -144,6 +144,15 @@ install:
       echo 'wxPython is available' ||
       echo 'wxPython is not available'
 
+    python -mpip install $PRE \
+        $PYTEST \
+        $PYTEST_COV \
+        pytest-faulthandler \
+        $PYTEST_PEP8 \
+        pytest-rerunfailures \
+        pytest-timeout \
+        pytest-xdist \
+        tox
   - |
     # Install matplotlib
     python -mpip install -ve .
@@ -160,6 +169,7 @@ 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

requirements/testing/travis_all.txt

@@ -17,3 +17,4 @@ pytest-xdist
 python-dateutil
 sphinx
 tornado
+tox

tox.ini

@@ -18,3 +18,11 @@ commands =
     pytest --pyargs matplotlib
 deps =
     pytest
+
+[testenv:pytz]
+changedir = /tmp
+commands =
+    pytest -m pytz {toxinidir}
+deps =
+    pytest
+    pytz