Merge pull request #162 from ConorMacBride/pin-mpl35

Pin tox environment `mpl35` to matplotlib 3.5.1

Author: Cadair

tox.ini

@@ -26,7 +26,7 @@ deps =
     mpl32: matplotlib==3.2.*
     mpl33: matplotlib==3.3.*
     mpl34: matplotlib==3.4.*
-    mpl35: matplotlib==3.5.*
+    mpl35: matplotlib==3.5.1
     mpldev: git+https://github.com/matplotlib/matplotlib.git#egg=matplotlib
 extras =
     test