Rework the HTTPS connection classes

[pquentin]

We realized in #1726 that all HTTPS connections go through the VerifiedHTTPSConnection class, even when they're unverified. This means we can merge VerifiedHTTPSConnection into HTTPSConnection.

However, those classes are part of our public API (see https://urllib3.readthedocs.io/en/latest/reference/index.html#module-urllib3.connection). The current layout is:

• HTTPSConnection is not visible, but is exported as UnverifiedHTTPSConnection
• VerifiedHTTPSConnection is available directly, but is also exported as HTTPSConnection

To preserve compatibility, we can expose HTTPSConnection directly, and then also export it as VerifiedHTTPSConnection and UnverifiedHTTPSConnection. This allows us to move to a single class while still exposing the same classes as before.

It will actually be an improvement, because HTTPSConnection exported as UnverifiedHTTPSConnection currently does not work as its connect() method uses variables that don't exist.

Another option is to stop exporting aliases, and only expose HTTPSConnection, but this is a breaking change. For example, it's likely to break vcrpy: https://github.com/kevin1024/vcrpy/blob/7caf29735ac6bae9fd17310460a00d66c3eb3a95/vcr/stubs/requests_stubs.py. So let's not do this.

(It would be interesting to make sure that vcr.py still works after this change.)

[pquentin]

Closing in favor of #1799.