Warnings in generate_packets.py

[lmoureaux]

Describe the bug
Python throws warnings when parsing generate_packets.py:

[228/509] Generating hand_gen.h, hand_gen.cpp
D:\a\freeciv21\freeciv21\common\generate_packets.py:131: SyntaxWarning: invalid escape sequence '\('
  mo = re.search("^(.*)\((.*)\)$", kind)
D:\a\freeciv21\freeciv21\common\generate_packets.py:136: SyntaxWarning: invalid escape sequence '\D'
  mo = re.search("^(\D+)(\d+)$", typeinfo["dataio_type"])
D:\a\freeciv21\freeciv21\common\generate_packets.py:191: SyntaxWarning: invalid escape sequence '\('
  mo = re.search("^add-cap\((.*)\)$", i)
D:\a\freeciv21\freeciv21\common\generate_packets.py:195: SyntaxWarning: invalid escape sequence '\('
  mo = re.search("^remove-cap\((.*)\)$", i)
D:\a\freeciv21\freeciv21\common\generate_packets.py:1077: SyntaxWarning: invalid escape sequence '\s'
  mo = re.search("^\s*(\S+)\s*=\s*(\d+)\s*;\s*(.*?)\s*$", lines[0])
D:\a\freeciv21\freeciv21\common\generate_packets.py:1159: SyntaxWarning: invalid escape sequence '\('
  mo = re.search("^cancel\((.*)\)$", i)
D:\a\freeciv21\freeciv21\common\generate_packets.py:1697: SyntaxWarning: invalid escape sequence '\s'
  lines = map(lambda x: re.sub("\s*#.*$", "", x), lines)
D:\a\freeciv21\freeciv21\common\generate_packets.py:1698: SyntaxWarning: invalid escape sequence '\s'
  lines = map(lambda x: re.sub("\s*//.*$", "", x), lines)
D:\a\freeciv21\freeciv21\common\generate_packets.py:1699: SyntaxWarning: invalid escape sequence '\s'
  lines = filter(lambda x: not re.search("^\s*$", x), lines)

To Reproduce
Steps to reproduce the behavior:

1. Use a recent Python (3.12.0 for the example above)
2. Build
3. Look at the logs
4. See warnings

Expected behavior
No warnings

Platform and version (please complete the following information):

• OS: Windows 10
• Freeciv21 version: Fix "migration due to not enough food" #2471

Additional context
This script underwent some refactoring in the Freeciv repo.