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`.
_osx_support.py
curses
dbm
ensurepip
idlelib
lib2to3
tkinter
turtle.py
turtledemo
venv
distutils/command/wininst-*.exe
webbrowser.py

_osx_support.py
curses
dbm
ensurepip
idlelib
lib2to3
tkinter
turtle.py
turtledemo
venv
distutils/command/wininst-*.exe
webbrowser.py
