chore: add note explaining google-api-core < 3.0.0dev pin (#454)

Expand pins on library dependencies in preparation for these dependencies taking a new major version. See googleapis/google-cloud-python#10566.

setup.py

@@ -30,7 +30,10 @@
 release_status = "Development Status :: 5 - Production/Stable"
 dependencies = [
     "grpcio >= 1.38.1, < 2.0dev",  # https://github.com/googleapis/python-pubsub/issues/414
-    "google-api-core[grpc] >= 1.26.0, < 2.0.0dev",
+    # NOTE: Maintainers, please do not require google-api-core>=2.x.x
+    # Until this issue is closed
+    # https://github.com/googleapis/google-cloud-python/issues/10566
+    "google-api-core[grpc] >= 1.26.0, <3.0.0dev",
     "libcst >= 0.3.10",
     "proto-plus >= 1.7.1",
     "grpc-google-iam-v1 >= 0.12.3, < 0.13dev",