Improve filename computation for syntax.dbg

[Technologicat]

Just to mark a future TODO.

The issue with using dbg[] in the REPL is that __file__ is not defined. There's a filename computation in the stdlib's warnings.py that addresses exactly this.

• Maybe we could adapt the solution for our purposes?
• Performance implications, may need to run the code at each call site of dbg? Is the REPL support for syntax.dbg worth it?

[Technologicat]

Something as simple as this could work:

def _callsite_filename():
     stack = inspect.stack()
     frame = stack[1].frame
     filename = frame.f_code.co_filename
     del frame, stack
     return filename

[Technologicat]

Issues cleanup: Now that I have a plan, this is a low-hanging fruit. Let's try to have this in 0.14.2.

[Technologicat]

And, since it's for debugging, this doesn't need high performance. So having to call inspect.stack for each invocation of dbg shouldn't be a problem.

Least surprise, with fewest caveats, is more important. Fixing this allows us to remove the note from the docs that dbg won't work in the REPL.