gh-155731: Clean up files left by test_import (GH-155735)

test_import() and test_unencodable_filename() left the __pycache__
directory created for a temporary module, and
test_import_in_del_does_not_crash() left the script it created.
(cherry picked from commit b0ba60e)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: serhiy-storchaka

Lib/test/test_import/__init__.py

@@ -487,6 +487,7 @@ def test_with_extension(ext):
                 forget(TESTFN)
                 unlink(source)
                 unlink(pyc)
+                rmtree('__pycache__')
 
         sys.path.insert(0, os.curdir)
         try:
@@ -666,6 +667,7 @@ def __del__(self):
                   import importlib
             sys.argv.insert(0, C())
             """))
+        self.addCleanup(unlink, testfn)
         script_helper.assert_python_ok(testfn)
 
     @skip_if_dont_write_bytecode
@@ -2063,6 +2065,7 @@ def test_unencodable_filename(self):
         # encode filenames, especially on Windows
         pyname = script_helper.make_script('', TESTFN_UNENCODABLE, 'pass')
         self.addCleanup(unlink, pyname)
+        self.addCleanup(rmtree, '__pycache__')
         name = pyname[:-3]
         script_helper.assert_python_ok("-c", "mod = __import__(%a)" % name,
                                        __isolated=False)