python3Package.python-markdown-math: remove support for python 2.7

As stated in its changelog [1], python 2.7 is no longer supported.

[1] https://github.com/mitya57/python-markdown-math/blob/master/changelog#L4

pkgs/development/python-modules/python-markdown-math/default.nix

@@ -2,11 +2,13 @@
 , buildPythonPackage
 , fetchPypi
 , markdown
+, isPy27
 }:
 
 buildPythonPackage rec {
   pname = "python-markdown-math";
   version = "0.7";
+  disabled = isPy27;
 
   src = fetchPypi {
     inherit pname version;