chore: expand range to allow 2.x versions (#768)

api-core, cloud-core, and resumable-media wil all be releasing Python3-only
2.x versions shortly.

Closes #767.

setup.py

@@ -30,10 +30,10 @@
 release_status = "Development Status :: 5 - Production/Stable"
 dependencies = [
     "grpcio >= 1.38.1, < 2.0dev",  # https://github.com/googleapis/python-bigquery/issues/695
-    "google-api-core[grpc] >= 1.29.0, < 2.0.0dev",
+    "google-api-core[grpc] >= 1.29.0, < 3.0.0dev",
     "proto-plus >= 1.10.0",
-    "google-cloud-core >= 1.4.1, < 2.0dev",
-    "google-resumable-media >= 0.6.0, < 2.0dev",
+    "google-cloud-core >= 1.4.1, < 3.0dev",
+    "google-resumable-media >= 0.6.0, < 3.0dev",
     "packaging >= 14.3",
     "protobuf >= 3.12.0",
     "requests >= 2.18.0, < 3.0.0dev",