Make scripts/docs/dot_converter.py Python 3 compatible so //site can be built with Bazel 0.27 without --force_host_python=PY2.
Make scripts/docs/dot_converter.py Python 3 compatible so //site can be built with Bazel 0.27 without --force_host_python=PY2.

Python 3 changed the type of subprocess#communicate to accept bytes, not str. Change everything else accordingly.

```
$ bazel build //site

# ...

Traceback (most recent call last):
  File "/usr/local/google/home/jingwen/.cache/bazel/_bazel_jingwen/c512c74f287f8aa09b4f3666e26c2b04/sandbox/linux-sandbox/777/execroot/io_bazel/bazel-out/host/bin/scripts/docs/generate_dot_graphs.runfiles/io_bazel/scripts/docs/generate_dot_graphs.py", line 23, in <module>
    converter.convert()
  File "/usr/local/google/home/jingwen/.cache/bazel/_bazel_jingwen/c512c74f287f8aa09b4f3666e26c2b04/sandbox/linux-sandbox/777/execroot/io_bazel/bazel-out/host/bin/scripts/docs/generate_dot_graphs.runfiles/io_bazel/scripts/docs/dot_converter.py", line 42, in convert
    output = dot.communicate(graph)[0].decode()
  File "/usr/lib/python3.6/subprocess.py", line 843, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
  File "/usr/lib/python3.6/subprocess.py", line 1499, in _communicate
    input_view = memoryview(self._input)
TypeError: memoryview: a bytes-like object is required, not 'str'
```

RELNOTES: None.
PiperOrigin-RevId: 254177933

katre/bazel,safarmer/bazel,katre/bazel,safarmer/bazel,werkt/bazel,perezd/bazel,twitter-forks/bazel,meteorcloudy/bazel,aehlig/bazel,safarmer/bazel,ulfjack/bazel,dslomov/bazel,cushon/bazel,werkt/bazel,ulfjack/bazel,werkt/bazel,akira-baruah/bazel,ulfjack/bazel,dslomov/bazel-windows,perezd/bazel,bazelbuild/bazel,ulfjack/bazel,dslomov/bazel,meteorcloudy/bazel,aehlig/bazel,ButterflyNetwork/bazel,aehlig/bazel,aehlig/bazel,bazelbuild/bazel,meteorcloudy/bazel,ButterflyNetwork/bazel,werkt/bazel,ulfjack/bazel,twitter-forks/bazel,davidzchen/bazel,davidzchen/bazel,meteorcloudy/bazel,katre/bazel,bazelbuild/bazel,cushon/bazel,aehlig/bazel,davidzchen/bazel,bazelbuild/bazel,ButterflyNetwork/bazel,cushon/bazel,perezd/bazel,dslomov/bazel,akira-baruah/bazel,werkt/bazel,cushon/bazel,dslomov/bazel-windows,ButterflyNetwork/bazel,twitter-forks/bazel,akira-baruah/bazel,akira-baruah/bazel,dslomov/bazel-windows,twitter-forks/bazel,twitter-forks/bazel,bazelbuild/bazel,davidzchen/bazel,twitter-forks/bazel,meteorcloudy/bazel,perezd/bazel,ulfjack/bazel,perezd/bazel,dslomov/bazel,akira-baruah/bazel,ButterflyNetwork/bazel,dslomov/bazel,katre/bazel,meteorcloudy/bazel,safarmer/bazel,perezd/bazel,katre/bazel,bazelbuild/bazel,davidzchen/bazel,dslomov/bazel,cushon/bazel,akira-baruah/bazel,dslomov/bazel-windows,perezd/bazel,twitter-forks/bazel,dslomov/bazel-windows,cushon/bazel,dslomov/bazel,davidzchen/bazel,safarmer/bazel,werkt/bazel,aehlig/bazel,dslomov/bazel-windows,aehlig/bazel,safarmer/bazel,katre/bazel,ButterflyNetwork/bazel,davidzchen/bazel,ulfjack/bazel,meteorcloudy/bazel