Add doc build requirements for RTD

[nateprewitt]

Adding a requirements.txt file under the doc directory to specify our doc generation requirements. The only current requirement is Sphinx>4, for #5954. Once this is merged, this can be wired up to Read The Docs to be used when auto-building changes. We're currently relying on the default which is Sphinx 1.8.2 and missing some more recent features.

[bhrutledge]

FYI @sigmavirus24 this relates back to pypa/twine#810 (comment).

[bhrutledge]

@nateprewitt Could y'all enable Preview Documentation from Pull Requests for RTD? Beyond being an additional helpful check for documentation-oriented PR's, I think it might let me test my projects to see if this change resolves the intersphinx issue.

[sigmavirus24]

I don't have access to the project on RTD and I'm thinking that @nateprewitt doesn't either. This certainly won't break things, so we'll merge it since we can't do PR changes

[nateprewitt]

@bhrutledge I'm looking at options to enable the preview builds. This change is probably worth doing regardless. I've enabled building with the new requirements, so the live docs should be set to test.

[cristianMuntean1]

from requests_oauthlib import OAuth1Session
twitter = OAuth1Session('client_key',
client_secret='client_secret',
resource_owner_key='resource_owner_key',
resource_owner_secret='resource_owner_secret')
url = 'https://api.twitter.com/1/account/settings.json'
r = twitter.get(url)