url: always use str for proxy configuration Previously, proxies didn't work on Python 3 for various reasons. First, the keys to the "proxies" dict are fed into a `setattr(self, "%s_open", ...)` call and passing bytestrings results in setting an oddly named attribute due to the b'' in %s formatting. This resulted in "http_open" and "https_open" not being properly overridden and proxies not being used. Second, the standard library was expecting proxy URLs to be str. And various operations (including our custom code in url.py) would fail to account for the str/bytes mismatch. This commit normalizes everything to str and adjusts our proxy code in url.py to account for the presence of str on Python 3. Differential Revision: https://phab.mercurial-scm.org/D5952