Remove references to setup.py

We now use a static setup.cfg instead, update __init__.py and
docs/RELEASE.md to point there.

Signed-off-by: Joshua Lock <[email protected]>

Author: joshuagl

docs/RELEASE.md

@@ -5,7 +5,7 @@
   tracked by git
 * Ensure `docs/CHANGELOG.md` contains a one-line summary of each [notable
   change](https://keepachangelog.com/) since the prior release
-* Update `setup.py` and `tuf/__init__.py` to the new version number vA.B.C
+* Update `setup.cfg` and `tuf/__init__.py` to the new version number vA.B.C
 * Test packaging, uploading to Test PyPI and installing from a virtual environment
   (ensure commands invoking `python` below are using Python 3)
   * Remove existing dist build dirs

tuf/__init__.py

@@ -1,5 +1,5 @@
 # This value is used in the requests user agent.
-# setup.py has it hard-coded separately.
+# setup.cfg has it hard-coded separately.
 # Currently, when the version is changed, it must be set in both locations.
 # TODO: Single-source the version number.
 __version__ = "0.19.0"