Using dictionary comprehensions in place of dict().

[dhermes]

This is now possible after dropping support for Python 2.6.

Also tweaking a few places dict(...) was used to copy a dictionary to just use .copy().

We also have a lot of dict(parse_qsl(...)) going on, maybe we should use parse_qs and just deal with the slightly different output.

Also, we have

google/cloud/storage/batch.py:204:        return dict(multi._headers), body
google/cloud/storage/batch.py:319:        msg_headers = dict(sub_message._headers)

@tseaver I seem to recall a discussion as to why this wouldn't clobber any repeated headers, but would like some clarification.

[tseaver]

LGTM. dict(parse_qsl(query_sting)) returns single values, rather than lists, with the last one winning in case of duplicates. Given that we only use it in tests, where we know there aren't dupes, it is a simpler, cleaner form than the list of values returned by parse_qs(query_string).

[dhermes]

@tseaver Indeed. It'd be nice if parse_qs had a flag to enforce uniqueness if desired