Add backwards-compatible POPSRegression import shim

Allows existing code using `from POPSRegression import POPSRegression`
to continue working after the rename to popsregression, with a
DeprecationWarning pointing users to the new import path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tomswinburne

POPSRegression.py

@@ -0,0 +1,13 @@
+# Backwards compatibility shim. Remove in a future release.
+import warnings
+
+warnings.warn(
+    "Importing from 'POPSRegression' is deprecated. "
+    "Use 'from popsregression import POPSRegression' instead.",
+    DeprecationWarning,
+    stacklevel=2,
+)
+
+from popsregression import POPSRegression  # noqa: F401, E402
+
+__all__ = ["POPSRegression"]

pyproject.toml

@@ -34,6 +34,9 @@ classifiers = [
 Homepage = "https://github.com/tomswinburne/popsregression"
 Issues = "https://github.com/tomswinburne/popsregression/issues"
 
+[tool.setuptools]
+py-modules = ["POPSRegression"]
+
 [tool.setuptools_scm]
 version_file = "popsregression/_version.py"