bazel: Envoy fails on Bazel CI with Bazel@HEAD

Hey @brandjon,

we are seeing a Python error with Bazel@HEAD that we are not seeing with Bazel 0.26.1

UnicodeEncodeError: 'ascii' codec can't encode character '\U0001f60e' in position 190: ordinal not in range(128)

I believe you updated the Python rules to pick python3 instead of python2 by default in master?

Executing tests from //test/extensions/filters/network/thrift_proxy:integration_test
-----------------------------------------------------------------------------
WARNING: Perftools heap leak checker is active -- Performance may suffer
[==========] Running 98 tests from 2 test suites.
[----------] Global test environment set-up.
[----------] 96 tests from TransportAndProtocol/ThriftConnManagerIntegrationTest
[ RUN      ] TransportAndProtocol/ThriftConnManagerIntegrationTest.Success/FramedBinary
ERROR:root:Unexpected exception in handler
Traceback (most recent call last):
  File "/var/lib/buildkite-agent/builds/bk-docker-spqt/bazel-downstream-projects/envoy/test/extensions/filters/network/thrift_proxy/driver/generated/example/Example.py", line 239, in process_execute
    result.success = self._handler.execute(args.input)
  File "/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/d1c957111778f6d978cec2005bbaba74/sandbox/linux-sandbox/4808/execroot/envoy/bazel-out/k8-fastbuild/bin/test/extensions/filters/network/thrift_proxy/integration_test.runfiles/envoy/test/extensions/filters/network/thrift_proxy/driver/server.py", line 33, in execute
    print("server: execute({0})".format(param))
UnicodeEncodeError: 'ascii' codec can't encode character '\U0001f60e' in position 190: ordinal not in range(128)
client exception: 6: Internal error
test/extensions/filters/network/thrift_proxy/integration_test.cc:197: Failure
Expected equality of these values:
  1U
    Which is: 1
  counter->value()
    Which is: 0
[  FAILED  ] TransportAndProtocol/ThriftConnManagerIntegrationTest.Success/FramedBinary, where GetParam() = (4-byte object <00-00 00-00>, 4-byte object <00-00 00-00>, false) (742 ms)