build: Single source version number

As of setuptools 46.4.0, one can accomplish single source version
number with
    version = attr: package.__version__
in setup.cfg: As long as setuptools simplified AST parser is able to
read the file, this works without actually importing anything.

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

pyproject.toml

@@ -1,6 +1,6 @@
 # Build-system section
 [build-system]
-requires = ["setuptools>=40.8.0", "wheel"]
+requires = ["setuptools>=46.4.0", "wheel"]
 build-backend = "setuptools.build_meta"
 
 # Black section

setup.cfg

@@ -1,6 +1,6 @@
 [metadata]
 name = tuf
-version = 1.0.0
+version = attr: tuf.__version__
 author = https://www.updateframework.com
 author_email = theupdateframework@googlegroups.com
 description = A secure updater framework for Python