CI: LinuxBuildDocs failing

[timothydijamco]

LinuxBuildDocs is failing for my PR #2577. I've also reproduced the error locally on master.

Most relevant part of the stack trace:

...
  File "/opt/conda/envs/ibis-env/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 1829, in should_suppress_value_header
    return (self.object == UNINITIALIZED_ATTR or
  File "/ibis/ibis/expr/api.py", line 612, in f
    other = as_value_expr(other)
  File "/ibis/ibis/expr/types.py", line 1265, in as_value_expr
    val = literal(val)
  File "/ibis/ibis/expr/types.py", line 1229, in literal
    'passing it explicitly with the `type` keyword.'.format(value)
TypeError: The datatype of value <object object at 0x7f93e0bbc270> cannot be inferred, try passing it explicitly with the `type` keyword.

Exception occurred:
  File "/ibis/ibis/expr/types.py", line 1229, in literal
    'passing it explicitly with the `type` keyword.'.format(value)
TypeError: The datatype of value <object object at 0x7f93e0bbc270> cannot be inferred, try passing it explicitly with the `type` keyword.

You can see the full stack trace in this failing job:
https://dev.azure.com/ibis-project/ibis/_build/results?buildId=3994&view=logs&j=8f09edc2-e3b7-52de-126a-0225c4f3efa1&t=e3ba5a33-21a6-5484-d0be-59baf122f79c

[timothydijamco]

I'm currently looking into it. So far, my guess based on the stack trace is that while building the docs, Sphinx is now comparing some Ibis expression(s) to some Sphinx constant UNINITIALIZED_ATTR. Ibis tries to create a Literal expression out of UNINITIALIZED_ATTR but cannot.

I think this new failure in our CI is triggered by a Sphinx version change.
Sphinx version in the most recent succeeding docs build:

sphinx                    3.3.1              pyhd8ed1ab_1    conda-forge

Sphinx version in the failing docs build:

sphinx                    3.4.0              pyhd8ed1ab_0    conda-forge

[timothydijamco]

Fairly sure the above is correct, and this change linked below is what is causing the issue. This is included in Sphinx 3.4.0, which was released a few days ago.
https://github.com/sphinx-doc/sphinx/pull/8500/files#diff-e43bdd6f8f37a12d2536e09e57c5e8999cb8de18b9c7ba49126f90576c4328acR1813

I'll open an issue in Sphinx in a bit

[jreback]

this is reported in multiple places already

just pin sphinx for now
3.4.1 should fix

[timothydijamco]

The docs build should succeed now that sphinx is pinned

I opened an issue on the sphinx side for the underlying issue (sphinx-doc/sphinx#8583), and we can unpin sphinx once it is resolved