Pin sphinx to <4.3

[jni]

Description

A new version of sphinx was released today, breaking our doc builds, example:

https://github.com/scikit-image/scikit-image/runs/4168761105?check_suite_focus=true#step:5:2721

Error reads:

Extension error (sphinx_gallery.docs_resolv):
Handler <function embed_code_links at 0x7f4faec58af0> for event 'build-finished' threw an exception (exception: list indices must be integers or slices, not str)

I propose we pin sphinx until we can get to the bottom of this, since @grlee77 and I are working on the 0.19 release and we don't want this issue to hold it up.

[jni]

Thanks @hmaarrfk, good changes. 😃

[hmaarrfk]

squash and merge? Sorry i've been getting really bad habbits of using Github's online interface.