fix: nightly package version

The package version may not include the "-nightly" suffix. Otherwise,
`pip install .` fails with:

    setuptools.extern.packaging.version.InvalidVersion: Invalid version: '...-nightly'

setup.py

@@ -26,7 +26,7 @@ ABOUT = load_about()
 
 setup(
     name="tutor-cairn",
-    version=ABOUT["__version__"],
+    version=ABOUT["__package_version__"],
     url="https://github.com/overhangio/tutor-cairn",
     project_urls={
         "Code": "https://github.com/overhangio/tutor-cairn",

tutorcairn/__about__.py

@@ -1,4 +1,5 @@
 __version__ = "16.0.3"
+__package_version__ = __version__
 
 # Handle version suffix for nightly, just like tutor core.
 __version_suffix__ = ""