Forbid use of cadata arg in pyOpenSSL backend

[MarSoft]

The class urllib3.contrib.pyopenssl.PyOpenSSLContext as a load_verify_locations() method. This method accepts cadata as its third parameter.
If this parameter is given then it wraps the provided certificate data in io.BytesIO() and passes to OpenSSL.SSL.Context.load_verify_locations() as cafile param.
But OpenSSL.SSL.Context.load_verify_locations does not support a file-like object for cafile param, it expects only file path.

This seems to break intentions of #1804.

[pquentin]

Hmm, this is a tricky one, thanks for pointing this out. As you said, pyOpenSSL does not support cadata, and only accepts cafile (a file path) or capath (a directory path).

While we could write the data in a temporary file, I don't think urllib3 shoud write any kind of data to the filesystem. I think the best option is here is to refuse to handle the cadata argument, just like #1804 does it with Python < 2.7.9.

@sethmlarson @sigmavirus24 Thoughts?

[sigmavirus24]

While we could write the data in a temporary file, I don't think urllib3 shoud write any kind of data to the filesystem

Agreed

I think the best option is here is to refuse to handle the cadata argument, just like #1804 does it with Python < 2.7.9.

Makes sense to me

[sethmlarson]

I'm also in agreement here, no filesystem hacks instead fail loudly.

[pquentin]

@MarSoft Would you be interested in working on this?

[IvanLauLinTiong]

pyOpenSSL is deprecated and will be removed in future release version 2.x (#2691).