bpo-39895: Move pathlib.Path.touch() implementation into the path accessor

[barneygale]

Path.touch() does a lot of os-specific /stuff/ that should probably live in the accessor. Perhaps most importantly, is calls os.close() on whatever accessor.open() returns, which is problematic for those wishing to write their own accessor that doesn't work on a file descriptor level.

This PR:

• Routes Path.touch() via the path's accessor, in conformity with most other Path methods.
• Adds an _NormalAccessor.touch() method
• Removes _NormalAccessor.utime() (which was only used by touch())
• Removes Path._raw_open() (which was only used by touch())

https://bugs.python.org/issue39895

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[barneygale]

I think this doesn't require a NEWS entry as it's an internal refactor, assuming that subclassing Path/Accessor isn't (yet) properly supported.

[pitrou]

LGTM. Could you please merge/rebase from master and fix the conflicts?

[barneygale]

Rebased

[barneygale]

@pitrou can this be merged? Thanks