chore: drop 3.9 as a supported Python version

pip version 23 and up with Python 3.9 was not able to
resolve the dependency correctly

pyproject.toml

@@ -7,7 +7,7 @@ name = "dandisets-linkml-status-tools"
 dynamic = ["version"]
 description = 'Tools for reporting the status of the translation of DANDI schemas from Pydantic definitions to LinkML definitions'
 readme = "README.md"
-requires-python = ">=3.9"
+requires-python = ">=3.10"
 license = "MIT"  # todo: Is this the correct license?
 keywords = []
 authors = [
@@ -16,7 +16,6 @@ authors = [
 classifiers = [
   "Development Status :: 4 - Beta",
   "Programming Language :: Python",
-  "Programming Language :: Python :: 3.9",
   "Programming Language :: Python :: 3.10",
   "Programming Language :: Python :: 3.11",
   "Programming Language :: Python :: 3.12",
@@ -46,7 +45,7 @@ path = "src/dandisets_linkml_status_tools/__about__.py"
 allow-direct-references = true
 
 [tool.hatch.envs.default]
-python = "3.9"
+python = "3.10"
 
 [tool.hatch.envs.test]
 dependencies = [