ENH: Add Enforce from PDF2.0 in viewer_preferences (py-pdf#2511)

Closes py-pdf#2509

In order to use it:

```
# to set it:
writer.viewer_preferences.enforce += writer.viewer_preferences.PRINT_SCALING
# to clear it:
writer.viewer_preferences.enforce = None
```

Author: pubpub-zz

pypdf/generic/_viewerpref.py

@@ -59,6 +59,12 @@ def _get_arr(self, key: str, deft: Optional[List[Any]]) -> NumberObject:
         return self.get(key, None if deft is None else ArrayObject(deft))
 
     def _set_arr(self, key: str, v: Optional[ArrayObject]) -> None:
+        if v is None:
+            try:
+                del self[NameObject(key)]
+            except KeyError:
+                pass
+            return
         if not isinstance(v, ArrayObject):
             raise ValueError("ArrayObject is expected")
         self[NameObject(key)] = v
@@ -69,6 +75,10 @@ def _get_int(self, key: str, deft: Optional[NumberObject]) -> NumberObject:
     def _set_int(self, key: str, v: int) -> None:
         self[NameObject(key)] = NumberObject(v)
 
+    @property
+    def PRINT_SCALING(self) -> NameObject:
+        return NameObject("/PrintScaling")
+
     def __new__(cls: Any, value: Any = None) -> "ViewerPreferences":
         def _add_prop_bool(key: str, deft: Optional[BooleanObject]) -> property:
             return property(
@@ -140,6 +150,8 @@ def _add_prop_int(key: str, deft: Optional[int]) -> property:
         cls.print_pagerange = _add_prop_arr("/PrintPageRange", None)
         cls.num_copies = _add_prop_int("/NumCopies", None)
 
+        cls.enforce = _add_prop_arr("/Enforce", ArrayObject())
+
         return DictionaryObject.__new__(cls)
 
     def __init__(self, obj: Optional[DictionaryObject] = None) -> None:

tests/test_writer.py

@@ -1807,6 +1807,14 @@ def test_viewerpreferences():
     writer.viewer_preferences.direction = "/R2L"
     assert len(writer.root_object["/ViewerPreferences"]) == 1
 
+    assert writer.viewer_preferences.enforce == []
+    assert "/Enforce" not in writer.viewer_preferences
+    writer.viewer_preferences.enforce += writer.viewer_preferences.PRINT_SCALING
+    assert writer.viewer_preferences["/Enforce"] == ["/PrintScaling"]
+    writer.viewer_preferences.enforce = None
+    assert "/Enforce" not in writer.viewer_preferences
+    writer.viewer_preferences.enforce = None
+
     del reader.trailer["/Root"]["/ViewerPreferences"]
     assert reader.viewer_preferences is None
     writer = PdfWriter(clone_from=reader)