chore: add constraints file

[busunkim96]

Add constraints file to test lower bounds.

[busunkim96]

• futures is excluded since it is only needed for 2.7.
• pytz is excluded because there is no range.
  

  python-api-core/setup.py

  Lines 31 to 45 in b51b7f5

  	dependencies = [
  	"googleapis-common-protos >= 1.6.0, < 2.0dev",
  	"protobuf >= 3.12.0",
  	"google-auth >= 1.21.1, < 2.0dev",
  	"requests >= 2.18.0, < 3.0.0dev",
  	"setuptools >= 34.0.0",
  	"six >= 1.13.0",
  	"pytz",
  	'futures >= 3.2.0; python_version < "3.2"',
  	]
  	extras = {
  	"grpc": "grpcio >= 1.29.0, < 2.0dev",
  	"grpcgcp": "grpcio-gcp >= 0.2.2",
  	"grpcio-gcp": "grpcio-gcp >= 0.2.2",
  	}

[tseaver]

Fix b0rken CI.

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[tseaver]

@googlebot I consent.