bpo-33053: -m now adds *starting* directory to sys.path (GH-6231) (#6236)

Historically, -m added the empty string as sys.path
zero, meaning it resolved imports against the current
working directory, the same way -c and the interactive
prompt do.

This changes the sys.path initialisation to add the
*starting* working directory as sys.path[0] instead,
such that changes to the working directory while the
program is running will have no effect on imports
when using the -m switch.

(cherry picked from commit d5d9e02dd3)

Lib/test/test_bdb.py

@@ -524,13 +524,13 @@ def run_test(modules, set_list, skip=None):
     test.id = lambda : None
     test.expect_set = list(gen(repeat(()), iter(sl)))
     with create_modules(modules):
-        sys.path.append(os.getcwd())
         with TracerRun(test, skip=skip) as tracer:
             tracer.runcall(tfunc_import)
 
 @contextmanager
 def create_modules(modules):
     with test.support.temp_cwd():
+        sys.path.append(os.getcwd())
         try:
             for m in modules:
                 fname = m + '.py'
@@ -542,6 +542,7 @@ def create_modules(modules):
         finally:
             for m in modules:
                 test.support.forget(m)
+            sys.path.pop()
 
 def break_in_func(funcname, fname=__file__, temporary=False, cond=None):
     return 'break', (fname, None, temporary, cond, funcname)