Add SSL CA certificate information to `pip debug`

[chrahunt]

What's the problem this feature will solve?

As described in #6720 (comment), pip may be using several sources of information for the CA certificate bundle to use for HTTPS requests. This makes it hard to debug user issues.

Describe the solution you'd like

In the output of pip debug we should include:

• the cert setting from the highest-priority pip configuration file (and the configuration file path) - on second thought the location doesn't matter much
• os.environ.get('REQUESTS_CA_BUNDLE')
• os.environ.get('CURL_CA_BUNDLE')
• pip._vendor.certifi.where()

This will provide insight into the CA certificate bundle in use for a given request, which can then be used in instructions to the user in conjunction with curl/openssl to submit an HTTP request independent of pip and rule out pip-specific issues.

Alternative Solutions

Do nothing.

Additional context

• pip 9.0.1: SLError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661) #4459
• --quiet suppresses ssl errors #4919
• pip SSL certificate errors #6335
• Document the behavior of --cert #6720
• SSL Certificate issue: Unable to install virtualenv #6915

[Pachwenko]

I think I can handle this one, but how would one get the highest-priority pip configuration file?

I see:

pip/src/pip/_internal/configuration.py

Line 102 in a0b75cc

class Configuration(object):

but it doesn't seem to know which file is highest priority

[pradyunsg]

Discussed this in person at PyCon India sprints.

Noting here for myself, that the suggested approach makes sense, since our configuration is cascading in a manner not handled by Configuration.

[chrahunt]

@Pachwenko I don't have the answer offhand, but if you want to do the others that would be totally fine. We can check them off in the original post as they get completed.

[pradyunsg]

@chrahunt I think it's best to leave the "get highest level file" details to a follow up PR since we'd need to keep track of more details than we're exposing publicly from the Configuration object currently.

[chrahunt]

Agreed, that's what I was implying in my comment (in case it wasn't clear). 👍