py: strip the value of PYTHON_ZSTANDARD_IMPORT_POLICY

I managed to tickle introducing whitespace in the value. Let's just
strip whitespace so harmless accidents _just work_.

Author: indygreg

docs/news.rst

@@ -12,6 +12,9 @@ Version History
   This fixes a nogil / multi-threaded compile error. (#201, #275)
 * A zstandard commit to fix qsort detection on BSD operating systems
   has been backported. (#272)
+* The `PYTHON_ZSTANDARD_IMPORT_POLICY` environment variable now has leading
+  and trailing whitespace stripped. Values like ` cffi` and `cffi ` are
+  now equivalent to `cffi`.
 * The CI jobs for building wheels have been overhauled to always use
   `cibuildwheel` and `uv` (where possible). This change should be backwards
   compatible. But wheel building for this project has historically been

zstandard/__init__.py

@@ -37,7 +37,9 @@
 # defining a variable and `setup.py` could write the file with whatever
 # policy was specified at build time. Until someone needs it, we go with
 # the hacky but simple environment variable approach.
-_module_policy = os.environ.get("PYTHON_ZSTANDARD_IMPORT_POLICY", "default")
+_module_policy = os.environ.get(
+    "PYTHON_ZSTANDARD_IMPORT_POLICY", "default"
+).strip()
 
 if _module_policy == "default":
     if platform.python_implementation() in ("CPython",):