试试看：

import re

text = "This is a sample sentence with bullets:\n    Item 1\n    Item 2\nThis is the next sentence. This is the third sentence\n."
punct = set('.,;!?\'":-')

def replace(g):
    if g[1] in punct or g[2] in punct:
        return f'{g[1]}{g[2]}'
    else:
        return f'{g[1]}.{g[2]}'

text = re.sub(r'(.)\n+(.)', replace, text.strip())
print(text)

图纸：

This is a sample sentence with bullets:    Item 1.    Item 2.This is the next sentence. This is the third sentence.

import re

result = re.sub(r'\n', '', re.sub(r'(?<=[^.,:;!?])\n(?=[^.,:;!?])', '.', input))

在这里，我们用.替换非标点符号包围的\n，然后用空字符串替换所有其他\n。
但你需要检查清单的非标点符号，以适合您的需要。