gdb/testsuite: reformat file with a more recent version of black

A Python file in my previous commit (5eb2254a1d) was formatted with
an older version of black, which gives slightly different results.

Reformat with a newer version of black.  This should make our
post-commit testing happy again.

No functional changes in this commit.

gdb/testsuite/gdb.base/pc-not-saved.py

@@ -35,7 +35,6 @@ def set_break_bt_here_frame_id(pc, cfa):
 
 
 class break_unwinding(Unwinder):
-
     """An unwinder for the function 'break_bt_here'.  This unwinder will
     claim any frame for the function in question, but doesn't provide
     any unwound register values.  Importantly, we don't provide a