> maybe a more elegant solution than changing the type conditional 
> on PGEN can be found

In pgen, the filename is only used to display the following warning, in indenterror():

   <filename>: inconsistent use of tabs and spaces in indentation

In pratical, this warning never occurs on Grammar/Grammar: this file doesn't use indentation at all, only continuation lines.

A better solution is maybe just to drop the filename for pgen. Anyway, pgen only compiles *one* file (Grammar/Grammar), so we don't need the input filename ;-)