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: N = 2**11 ; import numpy ; a = numpy.array(numpy.random.rand(N), dtype=complex)
# run: fft(a)

# pythran export fft(complex [])

import numpy as np


def fft(x):
    return np.fft(x)

# setup: N = 2**11 ; import numpy ; a = numpy.array(numpy.random.rand(N), dtype=complex)
# run: fft(a)

# pythran export fft(complex [])

import numpy as np


def fft(x):
    return np.fft(x)
