`pytest_unconfigure` can be called even if `pytest_configure` was not.
In order to avoid errors during unconfigure only perform the unconfigure
if the configure was performed. Add a counter which is incremented
before configure to track the number of times pytest started
initialization so that the config is not popped early from the stack.
`testdir` is deprecated. It's identical to `pytester`, except that
`testdir` returns `py.path` objects whereas `pytester` returns Python
stdlib `Path` objects. So I had to update usages of those paths.
`pytester` was added in pytest 6.2, which is our minimum supported
version.
Nate Clark