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: cycler
  version: 0.11.0
source:
  url: https://files.pythonhosted.org/packages/5c/f9/695d6bedebd747e5eb0fe8fad57b72fdf25411273a39791cde838d5a8f51/cycler-0.11.0-py3-none-any.whl
  sha256: 3a27e95f763a428a739d2add979fa7494c912a32c17c4c38c4d5f082cad165a3
test:
  imports:
    - cycler
requirements:
  run:
    - six

package:
  name: cycler
  version: 0.11.0
source:
  url: https://files.pythonhosted.org/packages/5c/f9/695d6bedebd747e5eb0fe8fad57b72fdf25411273a39791cde838d5a8f51/cycler-0.11.0-py3-none-any.whl
  sha256: 3a27e95f763a428a739d2add979fa7494c912a32c17c4c38c4d5f082cad165a3
test:
  imports:
    - cycler
requirements:
  run:
    - six
