fix(deps): add packaging requirement (#68)

Add packaging requirement. packaging.version
              is used for a version comparison in transports/base.py and is needed after the upgrade to gapic-generator-python 0.46.3

Author: busunkim96

setup.py

@@ -42,6 +42,7 @@
     install_requires=(
         "google-api-core[grpc] >= 1.22.2, < 2.0.0dev",
         "proto-plus >= 1.10.0",
+        "packaging >= 14.3",
     ),
     extras_requires={},
     python_requires=">=3.6",

testing/constraints-3.6.txt

@@ -7,3 +7,4 @@
 # Then this file should have foo==1.14.0
 google-api-core==1.22.2
 proto-plus==1.10.0
+packaging==14.3