TensorBoard 2.20.0

The 2.20 minor series tracks TensorFlow 2.20.

Features

• Compatible with Python 3.13 (see next point)
• Added a dependency to Pillow (https://python-pillow.github.io/), to replace use of deprecated standard library imghdr (removed in Python 3.13) (#7009, thanks @Zamanhuseyinli)
• Updated vendored version of bleach library to 6.2.0 and removed dependency on six library. (#7013)