build: fix Pip package creation to use Python 3 (#3673)
build: fix Pip package creation to use Python 3 (#3673)

Summary:
The `//tensorboard/pip_package` target currently doesn’t build on
machines where `virtualenv(1)` defaults to using Python 2, because the
`bdist_wheel` invocation can’t parse the syntax:

```
  File "/tmp/tensorboard-pip.QY3U1MVs3Y/tensorboard/data/experimental/base_experiment.py", line 24
    class BaseExperiment(metaclass=abc.ABCMeta):
                                  ^
SyntaxError: invalid syntax
```

Test Plan:
Running `bazel build //tensorboard/pip_package` previously failed on my
machine, even when in a Python 3 virtualenv, and now works.

wchargin-branch: build-pip-package-with-py3
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard