deps: add protobuf dependency (#306)

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Fixes #305

setup.py

@@ -34,6 +34,7 @@
     "google-cloud-core >= 1.4.1, < 2.0dev",
     "google-resumable-media >= 0.6.0, < 2.0dev",
     "six >=1.13.0,< 2.0.0dev",
+    "protobuf >= 3.12.0",
 ]
 extras = {
     "bqstorage": [

testing/constraints-3.6.txt

@@ -8,6 +8,7 @@ libcst==0.2.5
 llvmlite==0.34.0
 # pandas 0.23.0 is the first version to work with pyarrow to_pandas.
 pandas==0.23.0
+protobuf == 3.12.0
 proto-plus==1.10.0
 pyarrow==1.0.0
 python-snappy==0.5.4