ci: check lint with Black

Summary:
The Black binary only runs Python 3.6 and up, so we only lint in the
Python 3 CI build, but that’s perfectly fine. I’ve moved that build to
be first so that we can fail fast.

The preceding large sequence of change commits was generated with:

```
#!/bin/sh
set -eu
black .
docformatter -i -r .
black .  # should be a no-op
export GIT_AUTHOR_NAME="TensorBoard Gardener"
export GIT_AUTHOR_EMAIL="tensorboard-gardener@google.com"
# Commit each directory in order of longest-named directory to
# shortest-named, to force a topological ordering. Collapse some select
# subtrees to reduce the number of commits.
git diff --numstat \
    | awk '{ print $3 }' \
    | rev \
    | cut -d / -f 2- \
    | rev \
    | sed -e 's#/\(test\|demo\)$##' \
    | sed -e 's#^\(tensorboard/compat\)/.*#\1#' \
    | sed -e 's#^\(tensorboard/plugins/interactive_inference\)/.*#\1#' \
    | sed -e 's#^\(tensorboard/summary\)/.*#\1#' \
    | sort -u \
    | awk '{ print length($0), $0 }' \
    | sort -nrs \
    | awk '{ print $2 }' \
    | while read -r dir
do
    git add "${dir}"
    if git commit -m "black: reformat directory ${dir}" >/dev/null 2>&1; then
        printf 'reformat %s: ' "${dir}"
        git show --oneline --shortstat | tail -n 1
    else
        printf 'skip %s (covered by subtrees)\n' "${dir}"
    fi
done
```

Test Plan:
Running `black --check .` currently passes, as all files have been fixed
up. It takes about 0.3 seconds on my machine.

wchargin-branch: ci-black

Author: wchargin

.travis.yml

@@ -1,8 +1,8 @@
 dist: trusty
 language: python
 python:
-  - "2.7"
   - "3.6"
+  - "2.7"
 
 branches:
   only:
@@ -46,12 +46,14 @@ before_install:
 
 install:
   - elapsed "install"
+  - "PY3=\"$(python -c 'if __import__(\"sys\").version_info[0] > 2: print(1)')\""
   # Older versions of Pip sometimes resolve specifiers like `tf-nightly`
   # to versions other than the most recent(!).
   - pip install -U pip
   # Lint check deps.
   - pip install flake8==3.7.8
   - pip install yamllint==1.17.0
+  - if [ -n "${PY3}" ]; then pip install black==19.10b0; fi
   # TensorBoard deps.
   - pip install futures==3.1.1
   - pip install grpcio==1.24.3
@@ -87,6 +89,8 @@ before_script:
   - flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
   # Lint frontend code
   - yarn lint
+  # Lint backend code
+  - if [ -n "${PY3}" ]; then black --check .; fi
   # Lint .yaml docs files. Use '# yamllint disable-line rule:foo' to suppress.
   - yamllint -c docs/.yamllint docs docs/.yamllint
   # Make sure that IPython notebooks have valid Markdown.