Add marker support to pytest plugin

[kingbuzzman]

This patch fixes a very niche scenario:

1. When you have a fixture that depends on time.
2. When you need to modify the time before the test is executed.
3. AND be able to change the time while inside the test

import time

@pytest.fixture
def current_time():
     return time.time()

def test_normal(current_time):
    assert current_time > 1742943111.0

@pytest.mark.time_machine("2000-01-01")
def test_mod(current_time, time_machine):
    assert current_time == 946684800.0
    time_machine.shift(1)
    assert current_time == 946684800.0
    assert int(time.time()) == 946684801

---

Currently, the code fails:

    @time_machine.travel("2000-01-01")
    def test_mod(current_time, time_machine):
>       assert current_time == 946684800.0
E       assert 1742944978.9803169 == 946684800.0

If we ignore the ability of being able to modify a fixture before our test runs, we still get an error when trying to move the time along...

    @time_machine.travel("2000-01-01")
    def test_mod(current_time, time_machine):
        # assert current_time == 946684800.0
>       time_machine.shift(1)

test_file.py: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <time_machine.TimeMachineFixture object at 0xffff654ba2c0>, delta = 1

    def shift(self, delta: dt.timedelta | int | float) -> None:
        if self.traveller is None:
>           raise RuntimeError(
                "Initialize time_machine with move_to() before using shift()."
            )
E           RuntimeError: Initialize time_machine with move_to() before using shift().

---

Background: I am in the process of migrating pytest-freezegun to time-machine, in that migration I have a lot of code, specially testing celery tasks that rely on this methodology.

@pytest.mark.time_machine("2000-01-01")
def test_task(db_model_instances, time_machine):
    # Note: "db_model_instances" returns a model that has one or more 
    #       datetime fields that depends on the time set by the decorator
    task()
    assert thing == thing

    time_machine.shift(10)  # skip ahead X seconds
    task()
    assert thing == other_thing

This change would make my migration 100% smoother.

[kingbuzzman]

@adamchainz can you authorize my PR to run all tests, i want to see the coverage. Thanks!

[kingbuzzman]

@adamchainz can you explain to me what about this change you don't like? Maybe i can amend it?

[adamchainz]

My silence isn't rejection, just prioritisation. I like the feature. I will try do a sprint on Time Machine this month and integrate this PR.

[kingbuzzman]

For some reason 3.10 is is failing... Running tests here

[kingbuzzman]

Woohoo! back in action! Not sure why this worked before..

[kingbuzzman]

@adamchainz my bad, I didn't notice there were conflicts, and thus you couldn't merge it.

[kingbuzzman]

Anything else you need me to do for this PR?

[kingbuzzman]

@adamchainz thank you so much! Now I can start the freeze-gun migration! 🥳