DOC/CI: Doc build is failing with sphinx 2.1
The doc build is failing with:
```
Recursion error:
maximum recursion depth exceeded while calling a Python object

This can happen with very large or deeply nested source files.  You can carefully increase the default Python recursion limit of 1000 in conf.py with e.g.:
    import sys; sys.setrecursionlimit(1500)
```

I tried increasing the limit from 1,000 to 20,000 but still the same error, so I guess the problem is something else.

I think this is the first job that failed: https://travis-ci.org/pandas-dev/pandas/jobs/542714631 (PR #26697)