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`.
# setup: import numpy as np; r = np.arange(1000000, dtype=float)
# run: rosen(r)
import numpy as np

# pythran export rosen(float[])


def rosen(x):
    t0 = 100 * (x[1:] - x[:-1] ** 2) ** 2
    t1 = (1 - x[:-1]) ** 2
    return np.sum(t0 + t1)

# setup: import numpy as np; r = np.arange(1000000, dtype=float)
# run: rosen(r)
import numpy as np

# pythran export rosen(float[])


def rosen(x):
    t0 = 100 * (x[1:] - x[:-1] ** 2) ** 2
    t1 = (1 - x[:-1]) ** 2
    return np.sum(t0 + t1)
