Don't die on missing win32 resources on non-win32

src/python/grpcio/grpc/_cython/cygrpc.pyx

@@ -31,6 +31,7 @@ cimport cpython
 
 import pkg_resources
 import os.path
+import sys
 
 # TODO(atash): figure out why the coverage tool gets confused about the Cython
 # coverage plugin when the following files don't have a '.pxi' suffix.
@@ -50,6 +51,7 @@ cdef class _ModuleState:
   cdef bint is_loaded
 
   def __cinit__(self):
+    if 'win32' in sys.platform:
       filename = pkg_resources.resource_filename(
           'grpc._cython', '_windows/grpc_c.64.python')
       if not pygrpc_load_core(filename):