rename _imp initialization function to follow conventions (#5432)
When the C imp module became _imp in 6f44d66bc491bad5b8d897a68da68e009e27829d, the initialization function should have been renamed from PyInit_imp to PyInit__imp.
diff --git a/Include/import.h b/Include/import.h
index 26c4b1f..ac3fc3b 100644
--- a/Include/import.h
+++ b/Include/import.h
@@ -10,7 +10,7 @@
#ifndef Py_LIMITED_API
PyAPI_FUNC(_PyInitError) _PyImportZip_Init(void);
-PyMODINIT_FUNC PyInit_imp(void);
+PyMODINIT_FUNC PyInit__imp(void);
#endif /* !Py_LIMITED_API */
PyAPI_FUNC(long) PyImport_GetMagicNumber(void);
PyAPI_FUNC(const char *) PyImport_GetMagicTag(void);