Remove sphinx pin to rely on upstream constraints (#334)

# Description

Don't pin Sphinx directly, as we can rely on MyST and PyData Sphinx
Theme to pin it if necessary.

cc @Czaki

requirements.txt

@@ -1,4 +1,3 @@
-sphinx<6
 sphinx-autobuild
 sphinx-tabs
 sphinx-tags
@@ -9,7 +8,7 @@ sphinx-copybutton
 sphinx-gallery
 sphinx_autodoc_typehints==1.12.0
 myst-nb
-napari-sphinx-theme>=0.3.0.dev0
+napari-sphinx-theme>=0.3.0
 matplotlib
 lxml
 imageio-ffmpeg