Add a more explicit check and an explanation comment to check.py

Apparently this package made it to platforms where the CPython
internal hashing is neither Sip Hash 13 or 24. The comment should be
enough to understand why the simple check.py smoke test fails there.

Fixes #8.

check.py

@@ -6,10 +6,16 @@
 import siphash24
 
 # Python uses a randomized seed unless told otherwise
-assert int(os.environ['PYTHONHASHSEED']) == 0
+assert int(os.environ.get('PYTHONHASHSEED', '')) == 0
+
+# Ensure that Python is built to use Sip Hash as internal hashing function.
+# The FNV hashing function is used on platforms that require aligned memory
+# access for integers. See PEP-456 for details
+algorithm = sys.hash_info.algorithm
+assert algorithm in {'siphash13', 'siphash24'}
 
 # Python up to release 3.10 uses Sip Hash 13, Python 3.11 and later uses Sip Hash 13
-siphash = getattr(siphash24, sys.hash_info.algorithm)
+siphash = getattr(siphash24, algorithm)
 
 DATA = b'spam'