Fix docs build

[balopat]

Fixes #3200.

As we will deprecate Sphinx soon anyway, and I wanted to have this situation resolved asap, I decided to pin to 3.1.* version of sphinx instead of fighting the new warnings from 3.2.0.

[viathor]

Does docs build only fail with Sphinx 3.2.0?

[balopat]

Does docs build only fail with Sphinx 3.2.0?

That's what it looks like from this PR. Locally on my Mac it threw 3 other warnings on 3.1.2 as well, that I find very confusing why that doesn't happen on the CI:

/Users/balintp/dev/proj/Cirq/cirq/google/__init__.py:docstring of cirq.google.NAMED_GATESETS:1: WARNING: Inline strong start-string without end-string.
/Users/balintp/dev/proj/Cirq/docs/api.rst.rst:41: WARNING: autosummary: stub file not found 'cirq.qft'. Check your autosummary_generate setting.
/Users/balintp/dev/proj/Cirq/docs/api.rst:541:<autosummary>:1: WARNING: Inline strong start-string without end-string.

[balopat]

Huh, but I think we should put some docs on NAMED_GATESET in the end - sphinx generates the docs of dict() for it 🤯
https://cirq.readthedocs.io/en/latest/api.html#google - which probably changed in one version of sphinx (on my Mac 3.1.2) which now includes **kwargs which then causes sphinx to think there is an unclosed ** (strong formatting).

[balopat]

Added #3202 to track the sphinx issues