`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

從入口點 shlex.shlex 開始，重現回報的序列：將初始輸入消耗到 EOF，呼叫 push_source('b', None)，然後讀取下一個 token。完成的標誌是新推入的 source 被消耗，而不是回傳 EOF；為此序列新增或更新回歸測試涵蓋，並執行相關的 shlex 測試。