Fix syntax for Python version condition in requirements

Author: m-aciek

Doc/requirements.txt

@@ -11,7 +11,7 @@ docutils==0.17.1
 # Jinja version is pinned to a version compatible with Sphinx version <4.5.
 jinja2==3.0.3
 # Sphinx 3.4.3 uses imghdr which was removed from stdlib in Python 3.11
-standard-imghdr==3.13.0; python_version>3.10
+standard-imghdr==3.13.0; python_version>'3.10'
 
 blurb