`shlex` silently discards a sourced stream when the source filename ends at EOF

Bug description:

Summary

When shlex.shlex source inclusion is enabled and the filename following the source directive is the final token in the parent input, with no trailing whitespace or newline, the stream returned by sourcehook() is opened or created normally but all of its tokens are silently discarded. The included stream is then closed.

It contradicts the documented source-inclusion semantics, under which input should be read from the included stream until EOF before parsing resumes in the parent stream.

Minimal reproducer

import io
import shlex

lexer = shlex.shlex("include child", posix=True)
lexer.source = "include"

included = io.StringIO("from-child")
lexer.sourcehook = lambda filename: (filename, included)

print(list(lexer))
print(included.closed)

Actual behavior

[]
True

The from-child token is never returned, and the included stream is closed. No exception is raised, so callers cannot detect from the token stream that included input was skipped.

Expected behavior

['from-child']
True

The included stream should be parsed completely, closed when it reaches EOF, and only then should parsing resume in the parent stream.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-156892

首先运行最小 Python 复现程序，并检查 shlex.shlex 源码中 sourcehook() 附近的源包含逻辑以及输入结束处理。完成的标准是：被包含的流在关闭之前产生 'from-child'，同时解析仍能在父流中正确继续。