add asttokens library back, it's used in natspec.py

setup.py

@@ -91,6 +91,7 @@ def _global_version(version):
     py_modules=["vyper"],
     install_requires=[
         "cbor2>=5.4.6,<6",
+        "asttokens>=2.0.5,<3",
         "pycryptodome>=3.5.1,<4",
         "packaging>=23.1,<24",
         "importlib-metadata",