use development version of Sphinx

Sphinx-4.0 fails to correctly process attributes docstrings/doc comments.
Using unreleased Sphinx-4.1 will allow to properly document class attributes,
escpecially it is benefitial in the case of documenting inherited attributes.
22 jobs for enh/ci_sphinx in 11 minutes (queued for 4 seconds)
Status Name Job ID Coverage
  Build Env
manual build-env:conda #210807
allowed to fail manual
manual build-env:debian #210806
allowed to fail manual
manual build-env:default #210808
allowed to fail manual
manual build-env:ubuntu #210805
allowed to fail manual
 
  Build
passed build:debian #210810

00:02:38

passed build:latest #210813

00:01:59

passed build:no-extras #210812

00:02:28

passed build:stable #210811

00:02:31

passed build:ubuntu #210809

00:03:15

 
  Test
passed build PDF documentation #210822

00:06:46

passed build documentation #210821

00:05:38

passed check for broken links in doc #210823

00:05:10

passed check for dependencies installed #210815

00:00:34

passed check whitespace style #210814

00:00:36

passed test:debian #210819

00:02:00

passed test:latest #210820

00:02:16

passed test:no-extras #210817

00:01:25

passed test:stable #210816

00:02:46

passed test:ubuntu #210818

00:02:15

 
  Deploy
manual remove_coverage #210824
allowed to fail manual
passed remove_docs #210826
manual

00:00:28

passed upload documentation to the test server #210825

00:00:27