fix: MSVC issue

[henryiii]

Attempt to fix issue seen in #2821. Pulled out the non-changelog part of that PR too (adding pytest-timeout, corrected mypy config).

CC @YannickJadoul @rwgk @bstaletic

[YannickJadoul]

Cross-linking to #2754

[rwgk]

I didn't follow the conversations enough to know if this PR was expected to also fix the TSAN error reported under #2754, but I just tried, and I still see this with current master:

  WARNING: ThreadSanitizer: data race llvm/llvm-project/libcxx/include/streambuf:462:13 in std::__tsan::basic_streambuf<char, std::__tsan::char_traits<char> >::xsputn(char const*, long) (pybind11/tests/test_iostream)
  WARNING: ThreadSanitizer: data race llvm/llvm-project/libcxx/include/__string:388:56 in std::__tsan::char_traits<char>::copy(char*, char const*, unsigned long) (pybind11/tests/test_iostream)

[henryiii]

I saw the output issue again, but at least the hangup stopped, and the issue is rarer than it used to be. Issue was in one job out of 5-6 sets of builds.

[YannickJadoul]

Hmmm, so we're still missing something? :-(