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;lx,ly=(2**6,2**6);u=np.zeros([lx,ly],dtype=np.double);u[lx//2,ly//2]=1000.0;tempU=np.zeros([lx,ly],dtype=np.double)
# run: diffusion(u,tempU,100)

# pythran export diffusion(float [][], float [][], int)


def diffusion(u, tempU, iterNum):
    """
    Apply Numpy matrix for the Forward-Euler Approximation
    """
    mu = 0.1

    for _ in range(iterNum):
        tempU[1:-1, 1:-1] = u[1:-1, 1:-1] + mu * (
            u[2:, 1:-1]
            - 2 * u[1:-1, 1:-1]
            + u[0:-2, 1:-1]
            + u[1:-1, 2:]
            - 2 * u[1:-1, 1:-1]
            + u[1:-1, 0:-2]
        )
        u[:, :] = tempU[:, :]
        tempU[:, :] = 0.0

# setup: import numpy as np;lx,ly=(2**6,2**6);u=np.zeros([lx,ly],dtype=np.double);u[lx//2,ly//2]=1000.0;tempU=np.zeros([lx,ly],dtype=np.double)
# run: diffusion(u,tempU,100)

# pythran export diffusion(float [][], float [][], int)


def diffusion(u, tempU, iterNum):
    """
    Apply Numpy matrix for the Forward-Euler Approximation
    """
    mu = 0.1

    for _ in range(iterNum):
        tempU[1:-1, 1:-1] = u[1:-1, 1:-1] + mu * (
            u[2:, 1:-1]
            - 2 * u[1:-1, 1:-1]
            + u[0:-2, 1:-1]
            + u[1:-1, 2:]
            - 2 * u[1:-1, 1:-1]
            + u[1:-1, 0:-2]
        )
        u[:, :] = tempU[:, :]
        tempU[:, :] = 0.0
