Correct location for extracting main branch version (#207)

Author: kroenlein

gemd/__version__.py

@@ -1 +1 @@
-__version__ = "1.18.1"
+__version__ = "1.18.2"

scripts/validate_version_bump.py

@@ -18,7 +18,7 @@ def main():
         raise ValueError(f"Couldn't extract version from {version_path}") from e
 
     try:
-        with popen(f"git show main:setup.py", mode="r") as fh:
+        with popen(f"git show main:gemd/__version__.py", mode="r") as fh:
             old_version = extract_version(fh)
     except Exception as e:
         raise ValueError(f"Couldn't extract version from main branch") from e