If `HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings\ProxyServer` contains the string `http=host:123;https=host:456;ftp=host:789`, then getproxies_registry() should return

    {'http': 'http://host:123', 'https': 'http://host:456', 'ftp': 'http://host:789'}

for consistency with WinInet and Chromium, but it actually returns

    {'http': 'http://host:123', 'https': 'https://host:456', 'ftp': 'ftp://host:789'}

This bug has existed for a very long time (since Python 2.0.1 if not earlier), but it was exposed recently when urllib3 added support for HTTPS-in-HTTPS proxies in version 1.26. Before that, an `https` prefix on the HTTPS proxy url was silently treated as `http`, accidentally resulting in the correct behavior.

There are additional bugs in the treatment of single-proxy strings (the case when the string contains no `=` character).

The Chromium code for parsing the ProxyServer string can be found here: https://source.chromium.org/chromium/chromium/src/+/refs/tags/89.0.4353.1:net/proxy_resolution/proxy_config.cc;l=86

Below is my attempt at modifying the code from `getproxies_registry` to approximately match Chromium's behavior. I could turn this into a patch, but I'd like feedback on the corner cases first.

    if '=' not in proxyServer and ';' not in proxyServer:
        # Use one setting for all protocols.
        # Chromium treats this as a separate category, and some software
        # uses the ALL_PROXY environment variable for a similar purpose,
        # so arguably this should be 'all={}'.format(proxyServer),
        # but this is more backward compatible.
        proxyServer = 'http={0};https={0};ftp={0}'.format(proxyServer)

    for p in proxyServer.split(';'):
        # Chromium and WinInet are inconsistent in their treatment of
        # invalid strings with the wrong number of = characters. It
        # probably doesn't matter.
        protocol, addresses = p.split('=', 1)
        protocol = protocol.strip()

        # Chromium supports more than one proxy per protocol. I don't
        # know how many clients support the same, but handling it is at
        # least no worse than leaving the commas uninterpreted.
        for address in addresses.split(','):
            if protocol in {'http', 'https', 'ftp', 'socks'}:
                # See if address has a type:// prefix
                if not re.match('(?:[^/:]+)://', address):
                    if protocol == 'socks':
                        # Chromium notes that the correct protocol here
                        # is SOCKS4, but "socks://" is interpreted
                        # as SOCKS5 elsewhere. I don't know whether
                        # prepending socks4:// here would break code.
                        address = 'socks://' + address
                    else:
                        address = 'http://' + address

            # A string like 'http=foo;http=bar' will produce a
            # comma-separated list, while previously 'bar' would
            # override 'foo'. That could potentially break something.
            if protocol not in proxies:
                proxies[protocol] = address
            else:
                proxies[protocol] += ',' + address