Blacklist sphinx 3.4.2 because of Flask regressions

Using sphinx version 3.4.2 with Flask and documentation builds results
in an error:

  Exception occurred:
    File "/opt/stack/keystone/.tox/docs/lib/python3.8/site-packages/flask/globals.py", line 38, in _lookup_req_object
      raise RuntimeError(_request_ctx_err_msg)
  RuntimeError: Working outside of request context.

This issue has occured a few times with sphinx in the past and has been
reported upstream [0][1].

This commit blacklists sphinx 3.4.2 until we get a newer release with a fix.

[0] sphinx-doc/sphinx#8655
[1] sphinx-doc/sphinx#2796

Partial-Bug: 1910419

Change-Id: Ie80e45571efafe73a553ddd89c5be90fe97bea4d

global-requirements.txt

@@ -441,7 +441,7 @@ selenium  # Apache-2.0
 # Blacklist setuptools 36.2.0 due to https://github.com/pypa/setuptools/issues/1086
 # Blacklist setuptools 48.0.0, 49.0.0 due to https://github.com/pypa/setuptools/issues/2232
 setuptools!=24.0.0,!=34.0.0,!=34.0.1,!=34.0.2,!=34.0.3,!=34.1.0,!=34.1.1,!=34.2.0,!=34.3.0,!=34.3.1,!=34.3.2,!=36.2.0,!=48.0.0,!=49.0.0  # PSF/ZPL
-sphinx!=1.6.6,!=1.6.7,!=2.1.0,!=3.0.0  # BSD
+sphinx!=1.6.6,!=1.6.7,!=2.1.0,!=3.0.0,!=3.4.2  # BSD
 sphinx-testing  # BSD License
 sphinxcontrib-actdiag  # BSD
 sphinxcontrib-apidoc  # BSD