* Update 1.20.3-notes.rst
* Create 1.20.3-changelog.rst
* Fix typo in 1.20.2-notes.rst

[skip ci]

We were installing towncrier from the github master branch because there
had been no releases with the needed fixes. That has changed with the
recent release of versions 19.9.0 and 21.3.0.

Without any argument towncrier defaults to `build`.  But towncrier
added a `--version` argument to just print the towncrier version.

NumPy relies on passing `--version` which collides.  Even if
towncrier might revert the change, using `towncrier build` explicitly
just doesn't hurt.

Closes gh-18788

- Create the 1.20.2-changelog
- Update the 1.20.2-notes