On Windows 10.0.19045, the mypy hook is failing on pre-commit because of two [attr-defined] errors, which prevents me from staging stages. The problem was first reported in pytest-dev#11343. This PR solves the problem by adding `type ignore` comments to the erroring lines:

```
src\_pytest\compat.py:324: error: Module has no attribute "getuid"; maybe "getpid" or "getppid"?  [attr-defined]
testing\test_parseopt.py:294: error: Module has no attribute "getencoding"  [attr-defined]
```

…hub.com/WarrenTheRabbit/pytest into add-type-ignore-for-attr-defined-errors

The previous implementation required the caller
to have knowledge of its platform's `getuid` error
code. Although this had the benefit of replacing a
hard-coded error value with a descriptive variable,
the attempt at future proofing `get_user_id` was
flagged as a poor design decision by Nicoddemus:
to future proof the function, `get_user_id` should
determine the error flag internally.

However, future-proofing was not the reason
the issue was opened. The issue was opened to
resolve an `[attr-defined]` error on Windows
platforms caused by *mypy*'s handling of
code reachability through platform checks.

Over the course of the open issue, additional housekeeping
tasks were also undertaken:
- adding a comment that links to an explanation of
*mypy*'s platform and version checking requirements
- adding a variable name and comment to clarify
the reason for the conditional return `return uid if uid . . .`

It is misleading to say that None is returned because
the uid is unreliable. That implies that a uid of uknown
integrity was retrieved. More accurately, and across all
platforms, None indicates that no such id was determined.

The previous constant name implied that a user id was returned
 and that it cannot be relied upon. That is not the case. -1 is an
 error flag and the constant should identify it as such.

Returning None does not mean that all future calls to the underlying
platform will result in a return value of None. It is a certainty for win32
and Emscripten platforms but only a possibility for other platforms.
The documentation should only state that the *current* call to the
underlying platform was unsuccessful.