docs: add TODO(py3.10) markers for fromisoformat workaround

Make the .replace("Z", "+00:00") workaround discoverable by both
`rg TODO` and `rg "3.10|py3.10"` so it can be cleanly removed
when the project drops Python 3.10 support.

Author: splch

openapi_python_client/parser/properties/datetime.py

@@ -55,6 +55,7 @@ def convert_value(cls, value: Any) -> Value | None | PropertyError:
         if value is None or isinstance(value, Value):
             return value
         if isinstance(value, str):
+            # TODO(py3.10): fromisoformat() rejects "Z". Remove when 3.10 support ends.
             normalized = value.replace("Z", "+00:00")
             try:
                 datetime.datetime.fromisoformat(normalized)  # make sure it's a valid value

openapi_python_client/templates/property_templates/datetime_property.py.jinja

@@ -1,4 +1,5 @@
 {% macro construct_function(property, source) %}
+{# TODO(py3.10): fromisoformat() rejects "Z". Remove when 3.10 support ends. #}
 datetime.datetime.fromisoformat({{ source }}.replace("Z", "+00:00"))
 {% endmacro %}