Update py to 1.8.1

[pyup-bot]

This PR updates py from 1.5.4 to 1.8.1.

Changelog

1.8.1

==================

- Handle ``FileNotFoundError`` when trying to import pathlib in ``path.common``
on Python 3.4 (207).

- ``py.path.local.samefile`` now works correctly in Python 3 on Windows when dealing with symlinks.

1.8.0

==================

- add ``"importlib"`` pyimport mode for python3.5+, allowing unimportable test suites
to contain identically named modules.

- fix ``LocalPath.as_cwd()`` not calling ``os.chdir()`` with ``None``, when
being invoked from a non-existing directory.

1.7.0

==================

- fix 174: use ``shutil.get_terminal_size()`` in Python 3.3+ to determine the size of the
terminal, which produces more accurate results than the previous method.

- fix pytest-dev/pytest2042: introduce new ``PY_IGNORE_IMPORTMISMATCH`` environment variable
that suppresses ``ImportMismatchError`` exceptions when set to ``1``.

1.6.0

==================

- add ``TerminalWriter.width_of_current_line`` (i18n version of
``TerminalWriter.chars_on_current_line``), a read-only property
that tracks how wide the current line is, attempting to take
into account international characters in the calculation.

Links

• PyPI: https://pypi.org/project/py
• Changelog: https://pyup.io/changelogs/py/
• Docs: http://py.readthedocs.io/