Change GIL aquire functions to make it c-core thread safe

src/python/grpcio/grpc/_cython/_cygrpc/security.pxd.pxi

@@ -29,4 +29,4 @@
 
 
 cdef grpc_ssl_roots_override_result ssl_roots_override_callback(
-    char **pem_root_certs) with gil
+    char **pem_root_certs) nogil

src/python/grpcio/grpc/_cython/_cygrpc/security.pyx.pxi

@@ -33,12 +33,14 @@ import pkg_resources
 
 
 cdef grpc_ssl_roots_override_result ssl_roots_override_callback(
-    char **pem_root_certs) with gil:
-  temporary_pem_root_certs = pkg_resources.resource_string(
-      __name__.rstrip('.cygrpc'), '_credentials/roots.pem')
-  pem_root_certs[0] = <char *>gpr_malloc(len(temporary_pem_root_certs) + 1)
-  memcpy(
-      pem_root_certs[0], <char *>temporary_pem_root_certs,
-      len(temporary_pem_root_certs))
-  pem_root_certs[0][len(temporary_pem_root_certs)] = '\0'
+    char **pem_root_certs) nogil:
+  with gil:
+    temporary_pem_root_certs = pkg_resources.resource_string(
+        __name__.rstrip('.cygrpc'), '_credentials/roots.pem')
+    pem_root_certs[0] = <char *>gpr_malloc(len(temporary_pem_root_certs) + 1)
+    memcpy(
+        pem_root_certs[0], <char *>temporary_pem_root_certs,
+        len(temporary_pem_root_certs))
+    pem_root_certs[0][len(temporary_pem_root_certs)] = '\0'
+
   return GRPC_SSL_ROOTS_OVERRIDE_OK