fix: remove the true/false parameter from mod_gil_not_used

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

Author: henryiii

include/pybind11/pybind11.h

@@ -1263,11 +1263,14 @@ PYBIND11_NAMESPACE_END(detail)
 // Use to activate Py_MOD_GIL_NOT_USED.
 class mod_gil_not_used {
 public:
-    explicit mod_gil_not_used(bool flag = true) : flag_(flag) {}
+    mod_gil_not_used() : flag_(true) {}
     bool flag() const { return flag_; }
 
+    friend mod_gil_not_used mod_gil_used();
+
 private:
     bool flag_;
+    explicit mod_gil_not_used(bool flag) : flag_(flag) {}
 };
 
 inline mod_gil_not_used mod_gil_used() { return mod_gil_not_used(false); }