Prepare release 6.1.0

findpaths: fix regression causing incorrect rootdir to be determined
(cherry picked from commit b250c9d)

[6.1.x] findpaths: fix regression causing incorrect rootdir to be determined

Regressed in 6.1.0 in 62e249a.
The `x` is an `str` but is expected to be a `pathlib.Path`. Not caught
by mypy because `config.getini()` returns `Any`.

Fix by just removing the `bestrelpath` call:

- testpaths are always relative to the rootdir, it thus would be very
  unusual to specify an absolute path there.

- The code was wrong even before the regression: `py.path.local`'s
  `bestrelpath` function expects a `py.path.local`, not an `str`. But it
  had some weird `try ... except AttributeError` fallback which just
  returns the argument, i.e. it was a no-op. So there is no behavior
  change.

- It seems reasonable to me to just print the full path if that's what
  the ini specifies.

(cherry picked from commit 61f80a7)

[6.1.x] terminal: fix crash in header reporting when absolute testpaths is used