Python 3.8 - Update _check_keys to avoid Runtime Error when sanitizing response for keywords

[oldarmyc]

Issues: Runtime error sanitizing python keywords from dictionary
Fixes #1561

Problem: In Python 3.8 a Runtime error is thrown when sanitizing python keywords from response dictionary.

    def _check_keys(self, rdict):
        """Call this from _local_update to validate response keys

            disallowed server-response json keys:
            1. The string-literal '_meta_data'
            2. strings that are not valid Python 2.7 identifiers
            3. strings beginning with '__'.

            :param rdict: from response.json()
            :raises: DeviceProvidesIncompatibleKey
            :returns: checked response rdict
            """
        if '_meta_data' in rdict:
            error_message = "Response contains key '_meta_data' which is "\
                "incompatible with this API!!\n Response json: %r" % rdict
            raise DeviceProvidesIncompatibleKey(error_message)

>       for x in rdict:
E       RuntimeError: dictionary keys changed during iteration

Analysis: Changing the iteration to loop through a copy of the response dictionary and update the original, to avoid updating the dictionary while looping through it.

Tests: The following existing tests within bigip/test/unit/test_resource.py cover the proposed changes.
test_Resource__check_keys_python_keyword
test_Resource__check_for_python_keywords
test_Resource__check_for_python_keywords_recursive