Add Reminder to common.h, so that we will not forget to purge !WITH_THREAD branches when dropping Python 3.6

Author: rwgk

include/pybind11/detail/common.h

@@ -206,6 +206,7 @@
 #endif
 
 #include <Python.h>
+// Reminder: WITH_THREAD is always defined if PY_VERSION_HEX >= 0x03070000
 #if PY_VERSION_HEX < 0x03060000
 #    error "PYTHON < 3.6 IS UNSUPPORTED. pybind11 v2.9 was the last to support Python 2 and 3.5."
 #endif