Define basestring in Python 3

The Python builtin `basestring` has been removed from all [currently supported version of Python](https://devguide.python.org/#status-of-python-branches) so define `basestring` in Python3 so that line 60 does not raise a NameError at runtime.

Related task: https://github.com/v8/v8/pull/38

Bug: v8:10256
Change-Id: I087c561fff5a19aab1fec71e1ea0435cbfeca5d2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2069317Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66403}

tools/clusterfuzz/v8_foozzie_test.py

@@ -14,6 +14,11 @@ import v8_foozzie
 import v8_fuzz_config
 import v8_suppressions
 
+try:
+  basestring
+except NameError:
+  basestring = str
+
 BASE_DIR = os.path.dirname(os.path.abspath(__file__))
 FOOZZIE = os.path.join(BASE_DIR, 'v8_foozzie.py')
 TEST_DATA = os.path.join(BASE_DIR, 'testdata')