You can already do something like this, using either an import hook or a custom encoding.
Perhaps I should repost a link that I already included in an earlier, but rather terse, reply on this thread, about the specific "constant" example.
You will find various examples of adding custom keywords.
Syntactic changes would be needed. The way that your example would be normally processed by Python
is as follows:
First, the entire content of the file would be read, and broken up into tokens (individual words and symbols).
Next, these tokens would be interpreted according to Python's grammar: your notation would raise a SyntaxError.
This would stop the entire process.
To actually do the import "from mykeywords import ...", one needs to go beyond the parsing stage.
This is where a custom encoding or an import hook can help, by transforming the source into valid Python code
before it is parsed. More information available at the link given above.