check_language.py misc. problems

Some things I noted trying to run the script on the Vulkan repo:

* Seems to require python 3, which /usr/bin/env python may return python 2 in some environments. It's possible asking for 'env python3' would suffice. If run with python 2, exits immediately with no failure code. Python 2 is at EOL now and I am definitely not suggesting backwards compatibility, just that it execute the right Python version.
* Matches 'master' inside a link to another repository, e.g. https:...blob/master/README.md, and should not as that's not under control of the source document. Also matches the word in certain other contexts where it might not be appropriate to report - frex we have a Python script containing a list of branch names and metadata for them, which looks like

> 'master': [ 4000, 4999, 4448 ],

I'll note additional problems here as I encounter them, unless you want separate issues for them. I don't know to what degree you are willing to make this a more general-purpose tool vs. just running on Amber, but we're kinda hoping to leverage off this.