In working on some docs contributions, I've run into issues where docs builds are failing in CI differently than they're failing locally.

Locally, running `make venv` from `Docs/` results in Sphinx 2, whereas on Azure Pipelines, the docs are built with Sphinx 1.8.2 (https://github.com/python/cpython/blob/1d4b16051f8550fd7dada3670a3e83ae13b99d3b/.azure-pipelines/docs-steps.yml#L15).

When running with Sphinx 2, this error emerges when running `make suspicious` when a change is around that triggers suspicious code (such as this commit https://github.com/python/cpython/pull/12547/commits/9bde7faf6f051d4a7306ac8629d915ce069392f7):

Exception occurred:
  File "/Users/jaraco/code/public/cpython/Doc/tools/extensions/suspicious.py", line 154, in report_issue
    self.warn('[%s:%d] "%s" found in "%-.120s"' %
AttributeError: 'CheckSuspiciousMarkupBuilder' object has no attribute 'warn'

First, we probably want to make these processes consistent (define the dependencies in exactly one place).

Second, we should probably determine why the doc builds are failing on Sphinx 2 and update the `suspicious` code to support Sphinx 2.