MAINT improve isolation for micropip tests. I added a new `wheel_base` fixture which creates a new temporary directory and mocks `site.getsitepackages()` to return it. I also removed the `WHEEL_BASE` global variable from micropip and instead calculate it each time in `micropip.install`.
import sys
from pathlib import Path

sys.path.append(str(Path(__file__).parents[2] / "tools"))

from pytest_wrapper import remove_num_threads_option


def test_find_imports():
    args = ["-v", "-n", "3"]
    remove_num_threads_option(args)
    assert args == ["-v"]

    args = ["-v", "-n", "3", "-k", "firefox"]
    remove_num_threads_option(args)
    assert args == ["-v", "-k", "firefox"]

import sys
from pathlib import Path

sys.path.append(str(Path(__file__).parents[2] / "tools"))

from pytest_wrapper import remove_num_threads_option


def test_find_imports():
    args = ["-v", "-n", "3"]
    remove_num_threads_option(args)
    assert args == ["-v"]

    args = ["-v", "-n", "3", "-k", "firefox"]
    remove_num_threads_option(args)
    assert args == ["-v", "-k", "firefox"]
