Treat `.pyt` files as Python source

[Esri](https://github.com/esri) introduced [Python Toolboxes in ArcGIS 10.1](http://resources.arcgis.com/en/help/main/10.1/index.html#//001500000022000000). They are simply Python source code with a different extension. [A user requested syntax highlighting of `.pyt` files on GitHub](http://ideas.arcgis.com/ideaView?id=087E00000004m2eIAA). Should be a trivial merge. Thanks.