I don’t know if you can assign something to the word python, in particular. It must be tied to python binaries and it must be kind of complicated to change. What I do know is that you can create a alias for your command, something like
$ alias meu_comando = 'xvfb-run python'
The use would be $ meu_comando foo.py . Read about alias here
EDIT:
According to this link:
An alias can be created with the same name as the core name of a command (i.e., a command without any options or Arguments). In such a case, it is the alias that is called (i.e., Activated) first when the name is used, rather than the command with the same name.
Therefore, you can do what you want. I just don’t know if it’s a good idea...
I really liked to use the Mands but it was discontinued. https://docs.python.org/2/library/commands.html for python 3

import commands
result = commands.getstatusoutput("xvfb-run python foo.py")

– Valdinei Dos Santos