pytester: typing for spawn/spawn_pytest

[blueyed]

Waiting on davidhalter/jedi#1472.
Blocked by #6435.

[bluetech]

LGTM.

BTW, the code in spawn_pytest is pretty scary 😨 , in that it looks open to shell-injection. It would be better to use the args format instead of joining the arguments to a string. That's not related to this PR though, and maybe the input is always good - I didn't check.

        invoke = " ".join(map(str, self._getpytestargs()))
        cmd = "{} --basetemp={} {}".format(invoke, basetemp, string)
        return self.spawn(cmd, expect_timeout=expect_timeout)

[blueyed]

Yeah, I've made it using *args in my fork already - also to have it in line with the other APIs.
IIRC pexpect splits it itself for a string, and handles cmd, *args also.

I'll rebase this after #6435.

[blueyed]

Rebased on master.