[py][wpiutil] Add type caster for WPI_String (#8500)

This adds a type caster for `WPI_String` so that pybind11 can more
easily auto-convert between that and strings. This helps remove the need
to do things like
[this](f1d77244c3)
in the opmodes fixup

wpiutil/src/main/python/pyproject.toml

@@ -120,6 +120,10 @@ types = ["wpi::util::StringMap"]
 header = "wpi_ct_string_type_caster.h"
 types = ["wpi::util::ct_string"]
 
+[[tool.semiwrap.export_type_casters.wpiutil-casters.headers]]
+header = "wpi_string_type_caster.h"
+types = ["WPI_String"]
+
 [[tool.semiwrap.export_type_casters.wpiutil-casters.headers]]
 header = "wpystruct.h"
 types = ["WPyStruct"]

wpiutil/src/main/python/wpiutil/src/type_casters/wpi_string_type_caster.h

@@ -0,0 +1,34 @@
+#include <pybind11/pybind11.h>
+
+#include "wpi/util/string.h"
+
+namespace pybind11::detail {
+
+template <> 
+struct type_caster<WPI_String> {
+public:
+    PYBIND11_TYPE_CASTER(WPI_String, _("str"));
+
+    bool load(handle src, bool convert) {
+        if (!src) {
+            return false;
+        }
+
+        Py_ssize_t size = -1;
+        const char *str = PyUnicode_AsUTF8AndSize(src.ptr(), &size);
+        if (!str) {
+            PyErr_Clear();
+            return false;
+        }
+
+        value = WPI_String(str, static_cast<size_t>(size));
+        
+        return true;
+    }
+
+    static handle cast(const WPI_String& str, return_value_policy /* policy */, handle /* parent */) {
+        return PyUnicode_FromStringAndSize(str.str, str.len);
+    }
+};
+
+} // namespace pybind11::detail