Allow creating a new terminal when running python file

[brettcannon]

Expected behaviour

Clicking on the run button runs the file

Actual behaviour

Nothing happens if another file is already running

Steps to reproduce:

[NOTE: Self-contained, minimal reproducing code samples are extremely helpful and will expedite addressing your issue]

1. Run a python file which keeps running (like an API)
2. Open another file (like one sending a request to the API)
3. Try to run it

(Originally from @claell who was part of the botched feature request conversion.)

[brettcannon]

/cc @claell as we are moving our feature requests back into issues.