fix bug in GenericText

PyTeX/format/generic_text.py

@@ -1,3 +1,4 @@
+from __future__ import annotations
 from pathlib import Path
 from typing import Union, List, Optional
 
@@ -83,7 +84,7 @@ class GenericText:
         else:
             return None
 
-    def __add__(self, other):
+    def __add__(self, other: Union[None, GenericText, List[str]]) -> GenericText:
         if not self.has_value():
             return other
         if isinstance(other, GenericText):
@@ -93,6 +94,16 @@ class GenericText:
         else:
             return GenericText(self.text + other)
 
+    def __iadd__(self, other: Union[None, GenericText, List[str]]) -> GenericText:
+        if not self.has_value():
+            self.text = other
+        elif isinstance(other, GenericText):
+            if other.has_value():
+                self.text += other.text
+        else:
+            self.text += other
+        return self
+
     def __radd__(self, other):
         if other is None:
             return self