subprocess: `CalledProcessError.__str__` crashes when returncode is None

Calling str() on a CalledProcessError crashes if its returncode is None.

The __str__ method uses if self.returncode and self.returncode < 0 for the "died with a signal" case, and otherwise formats the return code with %d. When returncode is None, that first check is falsy, so it falls through to the %d branch, and %d can't format None, so it raises TypeError. Having the exception's own string representation blow up is a pretty bad way to fail.

import subprocess
err = subprocess.CalledProcessError(None, "cmd")
str(err)  # TypeError: %d format: a real number is required, not NoneType

The fix is to check for returncode is None before the %d branch and return a plain message instead.

Found while going through devdanzin's audit of the standard library, item 9: https://gist.github.com/devdanzin/3198710e3c0128fda5e0a7b4e0768e5f

Linked PRs

• gh-153971
• gh-155923
• gh-155924
• gh-155931

从 subprocess.CalledProcessError.str 开始，使用 None returncode 复现 issue 中的示例。为失败的表示添加覆盖，并确认 str(err) 返回一条普通消息而不会引发异常；开始前请查看已链接的 PRs，因为相关工作已经列在此 issue 下。