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`.
  # build
  cmake
  ruamel.yaml
  # lint
  pre-commit
  # testing
  build==0.7.0
  hypothesis
  pexpect
  pytest
  pytest-asyncio
  pytest-cov
  pytest-httpserver
  pytest-instafail
  pytest-rerunfailures
  pytest-xdist
  selenium==4.1.0
  # maintenance
  bump2version

  # build
  cmake
  ruamel.yaml
  # lint
  pre-commit
  # testing
  build==0.7.0
  hypothesis
  pexpect
  pytest
  pytest-asyncio
  pytest-cov
  pytest-httpserver
  pytest-instafail
  pytest-rerunfailures
  pytest-xdist
  selenium==4.1.0
  # maintenance
  bump2version
