fix: revert to silent fallback to pure-Python build (#93)

Closes #92.

Add a fix for a segfault on MacOS 11 ("Big Sur"):

- In `extend`, parse prior 'crc' value to 'extend' as 'unsigned long'

'PyArg_ParseTuple' w/ 'k' flag seems not to assign cleanly into a 'uint32_t'
variable on at least some MacOS / Python combinations.

Closes #96.

setup.py

@@ -57,22 +57,30 @@ def run(self):
 )
 
 
-if CRC32C_PURE_PYTHON:
+def build_pure_python():
     setuptools.setup(
         packages=["google_crc32c"],
         package_dir={"": "src"},
         ext_modules=[],
     )
+
+
+def build_c_extension():
+    setuptools.setup(
+        packages=["google_crc32c"],
+        package_dir={"": "src"},
+        ext_modules=[module],
+        cmdclass={"build_ext": BuildExtWithDLL},
+    )
+
+
+if CRC32C_PURE_PYTHON:
+    build_pure_python()
 else:
     try:
-        setuptools.setup(
-            packages=["google_crc32c"],
-            package_dir={"": "src"},
-            ext_modules=[module],
-            cmdclass={"build_ext": BuildExtWithDLL},
-        )
+        build_c_extension()
     except SystemExit:
-        if "CRC32C_PURE_PYTHON" not in os.environ:
+        if CRC32C_PURE_PYTHON_EXPLICIT:
             # If build / install fails, it is likely a compilation error with
             # the C extension:  advise user how to enable the pure-Python
             # build.
@@ -81,4 +89,10 @@ def run(self):
                 "To enable building / installing a pure-Python-only version, "
                 "set 'CRC32C_PURE_PYTHON=1' in the environment."
             )
-        raise
+            raise
+
+        logging.info(
+            "Compiling the C Extension for the crc32c library failed. "
+            "Falling back to pure Python build."
+        )
+        build_pure_python()

src/google_crc32c/_crc32c.c

@@ -6,14 +6,15 @@
 static PyObject *
 _crc32c_extend(PyObject *self, PyObject *args)
 {
+    unsigned long crc_input;
     uint32_t crc;
     const char *chunk;
     Py_ssize_t length;
 
-    if (!PyArg_ParseTuple(args, "ks#", &crc, &chunk, &length))
+    if (!PyArg_ParseTuple(args, "ky#", &crc_input, &chunk, &length))
         return NULL;
 
-    crc = crc32c_extend(crc, (const uint8_t*)chunk, length);
+    crc = crc32c_extend((uint32_t)crc_input, (const uint8_t*)chunk, length);
 
     return PyLong_FromUnsignedLong(crc);
 }
@@ -26,7 +27,7 @@ _crc32c_value(PyObject *self, PyObject *args)
     const char *chunk;
     Py_ssize_t length;
 
-    if (!PyArg_ParseTuple(args, "s#", &chunk, &length))
+    if (!PyArg_ParseTuple(args, "y#", &chunk, &length))
         return NULL;
 
     crc = crc32c_value((const uint8_t*)chunk, length);