Add a script entrypoint named mathics3. (#1551)

I'm always frustrated when starting mathics by uvx. I have to run `uvx
--from mathics3 mathics` to start mathics.

This commit make user able to execute mathics with uvx directly like
`uvx mathics3`.

Author: hzhangxyz

pyproject.toml

@@ -82,6 +82,7 @@ cython = [
 
 [project.scripts]
 mathics = "mathics.__main__:main"
+mathics3 = "mathics.__main__:main"
 
 [tool.setuptools]
 include-package-data = false