Return type "str" of "readline" incompatible with return type "bytes" in supertype "IOBase" [override]

Since https://github.com/python/typeshed/pull/4145 (released with mypy 0.790) mypy fails with:

import io

class MyStringIO(io.StringIO):
    def readline(self, size=None) -> str:
        return super().readline(size)

Return type "str" of "readline" incompatible with return type "bytes" in supertype "IOBase" [override]

According to https://github.com/python/typeshed/pull/4145#issuecomment-639543492 this is likely a bug in mypy.

(mypy 0.800+dev.f220ce50725ed7f117832b9f03aed4d1a9508e00 (current master))

使用 mypy 运行提供的 StringIO 复现程序，并调查针对 io.StringIO.readline 相对于 IOBase 的覆盖检查。为报告的错误添加回归测试覆盖，并验证该覆盖能够被接受，且不会引入相关的类型检查回归。