Gracefully fall back when version metadata is missing (#4324)

* Update __init__.py

* Update manim/__init__.py

Co-authored-by: Benjamin Hackl <devel@benjamin-hackl.at>

---------

Co-authored-by: Benjamin Hackl <devel@benjamin-hackl.at>

manim/__init__.py

@@ -1,9 +1,16 @@
 #!/usr/bin/env python
 from __future__ import annotations
 
-from importlib.metadata import version
+from importlib.metadata import PackageNotFoundError, version
 
-__version__ = version(__name__)
+# Use installed distribution version if available; otherwise fall back to a
+# sensible default so that importing from a source checkout works without an
+# editable install (pip install -e .).
+try:
+    __version__ = version(__name__)
+except PackageNotFoundError:
+    # Package is not installed; provide a fallback version string.
+    __version__ = "0.0.0+unknown"
 
 
 # isort: off