[package] use google-crc32c instead of crc32c (fixes: aiortc#528)

setup.py

@@ -21,9 +21,9 @@
     "aioice>=0.7.5,<0.8.0",
     "av>=8.0.0,<9.0.0",
     "cffi>=1.0.0",
-    "crc32c>=2.1",
     "cryptography>=2.2",
     'dataclasses; python_version < "3.7"',
+    "google-crc32c>=1.1",
     "pyee>=6.0.0",
     "pylibsrtp>=0.5.6",
 ]

src/aiortc/rtcsctptransport.py

@@ -22,7 +22,12 @@
     no_type_check,
 )
 
-from crc32c import crc32c
+# The `crc32c` package offers better performance but uses an LGPL license,
+# so we use `google-crc32c` by default.
+try:
+    from crc32c import crc32c
+except ImportError:
+    from google_crc32c import value as crc32c
 from pyee import AsyncIOEventEmitter
 
 from .exceptions import InvalidStateError

stubs/google_crc32c.pyi

@@ -0,0 +1 @@
+def value(data: bytes) -> int: ...