Fix PyPDF2 3.0+ compatibility issues in snailmail

Add compatibility wrapper classes for PdfFileWriter/PdfFileReader
to support both PyPDF2 2.x and 3.x versions.

Changes:
- Add PyPDF2 compatibility classes in snailmail/models/snailmail_letter.py
- Create comprehensive documentation in doc/PATCH_PDFWRITER.md
- Support for addPage, appendPagesFromReader, and getPage methods

Resolves PyPDF2.errors.DeprecationError: PdfFileWriter is deprecated
and was removed in PyPDF2 3.0.0.

🤖 assisted by claude

odoo-bringout-oca-ocb-snailmail/snailmail/models/snailmail_letter.py

@@ -4,7 +4,23 @@ import re
 import base64
 import io
 
-from PyPDF2 import PdfFileReader, PdfFileWriter
+try:
+    from PyPDF2 import PdfWriter, PdfReader
+    
+    # Create compatibility classes for PyPDF2 3.0+
+    class PdfFileWriter(PdfWriter):
+        def addPage(self, page):
+            return self.add_page(page)
+        
+        def appendPagesFromReader(self, reader, after_page_append=None):
+            return self.append_pages_from_reader(reader, after_page_append)
+    
+    class PdfFileReader(PdfReader):
+        def getPage(self, page_num):
+            return self.pages[page_num]
+
+except ImportError:
+    from PyPDF2 import PdfFileWriter, PdfFileReader
 from reportlab.platypus import Frame, Paragraph, KeepInFrame
 from reportlab.lib.units import mm
 from reportlab.lib.pagesizes import A4