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

@@ -16,7 +16,7 @@ with io.open(
 
 setup(
     name="tutor-discovery",
-    version=about["__version__"],
+    version=about["__package_version__"],
     url="https://docs.tutor.overhang.io/",
     project_urls={
         "Documentation": "https://docs.tutor.overhang.io/",

tutordiscovery/__about__.py

@@ -1,4 +1,5 @@
 __version__ = "16.0.0"
+__package_version__ = __version__
 
 # Handle version suffix for nightly, just like tutor core.
 __version_suffix__ = ""