Fix pyproject.toml license field (#2065)

Per PEP 621 this should be a table, not a string. This resolves failures
installing on systems with newer setuptools (v61.3.0 or newer:
https://setuptools.pypa.io/en/latest/history.html#v61-3-0).

Signed-off-by: Joshua Lock <jlock@vmware.com>

pyproject.toml

@@ -7,7 +7,7 @@ build-backend = "hatchling.build"
 name = "tuf"
 description = "A secure updater framework for Python"
 readme = "README.md"
-license = "MIT OR Apache-2.0"
+license = { text = "MIT OR Apache-2.0" }
 requires-python = ">=3.7"
 authors = [
   { email = "theupdateframework@googlegroups.com" },