pyproject.toml: limit sphinx_rtd_theme to <3.0.0

sphinx_rtd_theme deprecated `html_theme_path` and is now compatible with
docutils 0.21. We could drop `html_theme_path` on stable, but the switch
to a newer docutils version would break our Python 3.8 CI jobs due to
the rst2man change [1]. So for stable, simply limit the sphinx_rtd_theme
dependency to <3.0.0. For master, see [1].

[1] #1519

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

pyproject.toml

@@ -52,7 +52,7 @@ dynamic = ["version"]  # via setuptools_scm
 
 [project.optional-dependencies]
 doc = [
-    "sphinx_rtd_theme>=1.0.0",
+    "sphinx_rtd_theme>=1.0.0,<3.0.0",
     "Sphinx>=2.0.0",
 ]
 docker = ["docker>=5.0.2"]
@@ -86,7 +86,7 @@ deb = [
 dev = [
     # references to other optional dependency groups
     # labgrid[doc]
-    "sphinx_rtd_theme>=1.0.0",
+    "sphinx_rtd_theme>=1.0.0,<3.0.0",
     "Sphinx>=2.0.0",
 
     # labgrid[docker]