Add `pybind11/stl.h` again.

... and make sure it stays.

Change-Id: Iab9f9e9445545a9c97aefd4d83691a70fde2b1c8
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63111
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

python/_re2.cc

diff --git a/python/_re2.cc b/python/_re2.cc
index 385492d..22f092b 100644
--- a/python/_re2.cc
+++ b/python/_re2.cc
@@ -17,6 +17,7 @@
 #include "pybind11/gil.h"
 #include "pybind11/pybind11.h"
 #include "pybind11/pytypes.h"
+#include "pybind11/stl.h"  // IWYU pragma: keep
 #include "re2/filtered_re2.h"
 #include "re2/re2.h"
 #include "re2/set.h"