`shlex.push_source()` fails to reset lexer state, causing new stream to be ignored if called after EOF

Bug report

Bug description:

Calling shlex.push_source() after the current input has reached EOF leaves the lexer in state == None, so the newly pushed source is never read. Subsequent get_token() calls immediately return EOF instead of consuming the new input.

from shlex import shlex

lexer = shlex('a')
print(lexer.get_token())
print(lexer.get_token())  # EOF -> state is None

lexer.push_source('b', None)
print(lexer.get_token())  # actural '' (EOF), expected: 'b'

a

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-144560
• gh-150172

从入口点 shlex.shlex 开始，复现所报告的序列：将初始输入消费到 EOF，调用 push_source('b', None)，然后读取下一个 token。完成的标志是新推入的 source 被消费，而不是返回 EOF；为此序列添加或更新回归覆盖，并运行相关的 shlex 测试。