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`.
package:
  name: python-dateutil
  version: 2.8.2
source:
  sha256: 961d03dc3453ebbc59dbdea9e4e11c5651520a876d0f4db161e8674aae935da9
  url: https://files.pythonhosted.org/packages/36/7a/87837f39d0296e723bb9b62bbb257d0355c7f6128853c78955f57342a56d/python_dateutil-2.8.2-py2.py3-none-any.whl
requirements:
  run:
    - six
test:
  imports:
    - dateutil

package:
  name: python-dateutil
  version: 2.8.2
source:
  sha256: 961d03dc3453ebbc59dbdea9e4e11c5651520a876d0f4db161e8674aae935da9
  url: https://files.pythonhosted.org/packages/36/7a/87837f39d0296e723bb9b62bbb257d0355c7f6128853c78955f57342a56d/python_dateutil-2.8.2-py2.py3-none-any.whl
requirements:
  run:
    - six
test:
  imports:
    - dateutil
