prevent Pdb from setting a break point with the "step" command after the last line

[TaiSakuma]

If the Pdb command step is used after the last line of the script, Pdb will set the trace_dispath function as the trace function of the caller's frame. Consequently, the trace_dispath function will be called at line 48 after the script execution has finished.

nextline/nextline/main.py

Lines 45 to 49 in a0ed6aa

	try:
	exec(cmd)
	finally:
	sys.settrace(trace_org)
	threading.settrace(trace_org)

The trace_dispath function is set in Bdb.set_trace()

https://github.com/python/cpython/blob/v3.9.5/Lib/bdb.py#L298-L308

[TaiSakuma]

Bdb.set_trace() should be intervened when the command step is used on the event return at the last line of the script.

[TaiSakuma]

The commit c93e93f is a quick fix.