Add input types to chrono type casters (#931)

The type casters in `stl/chrono.h` accept various types in their `from_python` calls, but they currently just use a name for their output type.

This uses `io_name` to differentiate, adding the various acceptable
types that the casters permit at runtime

include/nanobind/stl/chrono.h

@@ -99,7 +99,13 @@ template <typename type> class duration_caster {
         return pack_timedelta(dd.count(), ss.count(), us.count());
     }
 
-    NB_TYPE_CASTER(type, const_name("datetime.timedelta"))
+    #if PY_VERSION_HEX < 0x03090000
+        NB_TYPE_CASTER(type, io_name("typing.Union[datetime.timedelta, float]",
+                                     "datetime.timedelta"))
+    #else
+        NB_TYPE_CASTER(type, io_name("datetime.timedelta | float",
+                                     "datetime.timedelta"))
+    #endif
 };
 
 template <class... Args>
@@ -208,7 +214,13 @@ class type_caster<std::chrono::time_point<std::chrono::system_clock, Duration>>
                              localtime.tm_sec,
                              (int) us.count());
     }
-    NB_TYPE_CASTER(type, const_name("datetime.datetime"))
+    #if PY_VERSION_HEX < 0x03090000
+        NB_TYPE_CASTER(type, io_name("typing.Union[datetime.datetime, datetime.date, datetime.time]",
+                                     "datetime.datetime"))
+    #else
+        NB_TYPE_CASTER(type, io_name("datetime.datetime | datetime.date | datetime.time",
+                                     "datetime.datetime"))
+    #endif
 };
 
 // Other clocks that are not the system clock are not measured as