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`.
queries:
  - exclude: py/import-and-import-from

extraction:
  cpp:
    prepare:
      packages:
        - python3-yaml

queries:
  - exclude: py/import-and-import-from

extraction:
  cpp:
    prepare:
      packages:
        - python3-yaml
