build(deps): add 'validators' to install_requires (canonical#4898)

`validators` was added in canonical#4888 to the requirement files but 
not to `setup.py`

Signed-off-by: Callahan Kovacs <callahan.kovacs@canonical.com>

setup.py

@@ -132,6 +132,7 @@ def recursive_data_files(directory, install_directory):
     "tinydb",
     "typing-extensions",
     "urllib3<2",  # requests-unixsocket does not yet work with urllib3 v2.0+
+    "validators>=0.28.3",
 ]
 
 docs_requires = {