notebook: fix `cgi.escape` for Python 3.8 (#3016)
notebook: fix `cgi.escape` for Python 3.8 (#3016)

The deprecated `cgi.escape` function is removed in Python 3.8, in favor
of `html.escape`, which exists in Python 3.2+. This commit updates the
notebook integration to use the appropriate endpoint in all Pythons.

Fixes #3015.

Test Plan:
Run `bazel run //tensorboard/pip_package:extract_pip_package`, then
install the built wheels into fresh Python 2 and 3 virtualenvs with
TensorFlow and Jupyter installed. Run

```python
%load_ext tensorboard
%tensorboard --logdir /tmp/whatever

import sys
print("html" in sys.modules)
print("cgi" in sys.modules)
```

in each, and note that in each case the notebook extensions work, while
the imported modules are different in the two environments: `False True`
in Python 2, `True False` in Python 3.

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard