Update _base.py (#2325)

Since b"" is not mutable it causes python to allocate and deallocate memory repeatedly in the for loop which cause hang/long runtime when handle very large string.

For example when using add_js(javascript) to  to add a very big `javascript` bytestring.

pypdf/generic/_base.py

@@ -727,7 +727,7 @@ def readFromStream(
 
 
 def encode_pdfdocencoding(unicode_string: str) -> bytes:
-    retval = b""
+    retval = bytearray()
     for c in unicode_string:
         try:
             retval += b_(chr(_pdfdoc_encoding_rev[c]))