Improve handling of skip for module level

This is potentially about updating docs, updating error messages or introducing a new API.

Consider the following scenario:

pos_only.py is using Python 3,8 syntax:

def foo(a, /, b):
    return a + b

It should not be tested under Python 3.6 and 3.7. This is a proper way to skip the test in Python older than 3.8:

from pytest import raises, skip
import sys
if sys.version_info < (3, 8):
    skip(msg="Requires Python >= 3.8", allow_module_level=True)

# import must be after the module level skip:
from pos_only import *

def test_foo():
    assert foo(10, 20) == 30
    assert foo(10, b=20) == 30
    with raises(TypeError):
        assert foo(a=10, b=20)

My actual test involves parameterize and a 3.8 only class, so skipping the test itself is not sufficient because the 3.8 class was used in the parameterization.

A naive user will try to initially skip the module like:

if sys.version_info < (3, 8):
    skip(msg="Requires Python >= 3.8")

This issues this error:

Using pytest.skip outside of a test is not allowed. To decorate a test function, use the @pytest.mark.skip or @pytest.mark.skipif decorators instead, and to skip a module use `pytestmark = pytest.mark.{skip,skipif}.

The proposed solution pytestmark = pytest.mark.{skip,skipif}, does not work in my case: pytest continues to process the file and fail when it hits the 3.8 syntax (when running with an older version of Python).

The correct solution, to use skip as a function is actively discouraged by the error message.

This area feels a bit unpolished. A few ideas to improve:

1. Explain skip with allow_module_level in the error message. this seems in conflict with the spirit of the message.
2. Create an alternative API to skip a module to make things easier: skip_module("reason"), which can call _skip(msg=msg, allow_module_level=True).

[nicoddemus]

I agree it would be healthier, but -1 from me for the same reasons as @The-Compiler: we already had a deprecation/change period in order to introduce allow_module_level, having yet another one is frustrating/confusing to users, in comparison to the small gains.

[The-Compiler]

-0.5 from my side - I think this is too minor to warrant another deprecation and change.