使用最后支持Ubuntu 18.04的Pwndbg版本

[Shawdox]

Hi giantbranch!
I find that Pwndbg has drop the support for python 3.6 and 3.7 since Mar. 2023, which means its newest version has not support Ubuntu 18.04. So I change the pwndbg to the latest of Ubuntu 18.04-support version.

References:

• Drop support for Python 3.6 (Ubuntu 18.04) and Python 3.7 pwndbg/pwndbg#1744 (comment)
• Update code to Python >= 3.8 pwndbg/pwndbg#1840

[Shawdox]

并且豆瓣的pip源已经不再管用，这里建议换成清华的源：
echo -e "[global]\nindex-url = https://pypi.tuna.tsinghua.edu.cn/simple" > ~/.pip/pip.conf