Use standard setuptools attr: feature, fixing Python 3.14 compat

Use the standard setuptools `attr:` feature to obtain the version
via setuptools' built-in AST parser rather than writing a custom AST
parser in `setup.py`.  This fixes compatibility with Python 3.14,
since the custom parser is not compatible with the new `ast.Constant`
type.

Author: mgorny

setup.cfg

@@ -1,5 +1,6 @@
 [metadata]
 name = curtsies
+version = attr: curtsies.__version__
 description = Curses-like terminal wrapper, with colored strings!
 long_description = file: README.md,
 long_description_content_type = text/markdown