tests: fix builtin module test on pypy

On pypy datetime and cProfile are modules written in Python, not in C.
For the purpose of this test, just list them explicitely as builtins,
which silences warnings about them being imported before stdlib modules.

contrib/import-checker.py

@@ -184,6 +184,8 b' def list_stdlib_modules():'
     yield 'builtins' # python3 only
     for m in 'fcntl', 'grp', 'pwd', 'termios':  # Unix only
         yield m
+    for m in 'cPickle', 'datetime': # in Python (not C) on PyPy
+        yield m
     stdlib_prefixes = set([sys.prefix, sys.exec_prefix])
     # We need to supplement the list of prefixes for the search to work
     # when run from within a virtualenv.