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: MarkupSafe
  version: 2.1.1
source:
  sha256: 7f91197cc9e48f989d12e4e6fbc46495c446636dfc81b9ccf50bb0ec74b91d4b
  url: https://files.pythonhosted.org/packages/1d/97/2288fe498044284f39ab8950703e88abbac2abbdf65524d576157af70556/MarkupSafe-2.1.1.tar.gz
test:
  imports:
    - markupsafe

package:
  name: MarkupSafe
  version: 2.1.1
source:
  sha256: 7f91197cc9e48f989d12e4e6fbc46495c446636dfc81b9ccf50bb0ec74b91d4b
  url: https://files.pythonhosted.org/packages/1d/97/2288fe498044284f39ab8950703e88abbac2abbdf65524d576157af70556/MarkupSafe-2.1.1.tar.gz
test:
  imports:
    - markupsafe
