build: fix readme content type warning

Author: regisb

setup.py

@@ -41,6 +41,7 @@ def load_about():
     maintainer_email="[email protected]",
     description="Web-based user interface plugin for Tutor",
     long_description=load_readme(),
+    long_description_content_type="text/x-rst",
     packages=find_packages(exclude=["tests*"]),
     include_package_data=True,
     python_requires=">=3.8",