Fix building the documentation

The support for `Sphinx` version 7 has been implemented in
`rdt-themes`, but hasn't yet been released [1]. Currently, the build
uses `Sphinx` version 7 which fails with [2].

Use a `Sphinx` version prior to 7 in order to fix that issue.

[1] readthedocs/sphinx_rtd_theme#1463
[2] `UndefinedError("'style' is undefined")`

Signed-off-by: Frank Viernau <frank_viernau@epam.com>

Author: fviernau

requirements_builder.ABOUT

@@ -87,6 +87,6 @@ testing =
     pycodestyle
 
 docs =
-    Sphinx >= 3.3.1
+    Sphinx >= 3.3.1,<7.0.0
     sphinx-rtd-theme >= 0.5.0
     doc8 >= 0.8.1

setup.cfg

@@ -88,6 +88,6 @@ testing =
     pytest-rerunfailures
 
 docs =
-    Sphinx >= 3.3.1
+    Sphinx >= 3.3.1,<7.0.0
     sphinx-rtd-theme >= 0.5.0
     doc8 >= 0.8.1