ensure requirements pinned in tox.ini are not overridden

Author: ConorMacBride

tox.ini

@@ -18,6 +18,8 @@ setenv =
     mpldev: PIP_EXTRA_INDEX_URL = https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
 changedir = .tmp/{envname}
 description = run tests
+constrain_package_deps =
+    !mpldev-!pytestdev: true
 deps =
     pytest-xdist
     mpl20: matplotlib==2.0.*