devel/py-pytest: Update to 3.6.2

https://github.com/pytest-dev/pytest/blob/master/CHANGELOG.rst

• Update PORTVERSION and distinfo checksum to 3.6.2
• Add devel/py-more-itertools [0] and devel/py-atomicwrites [1] to RUN_DEPENDS
• Add shells/bash [2] to TEST_DEPENDS
• Change test target to run pytest, setup.py test has been removed [3]

[0] https://github.com/pytest-dev/pytest/commit/25399da9047d044174f090f1315f9a4244b91135
[1] https://github.com/pytest-dev/pytest/commit/aa95a425d7708d80fc887b1825662daf3a624ba7
[2] https://github.com/pytest-dev/pytest/blob/master/testing/test_argcomplete.py
[3] https://github.com/pytest-dev/pytest/commit/c1d9ca81df5364417317fd779e73d9ff85fe5d35

Reviewed by: koobs
Approved by: koobs (mentor)
Differential Revision: https://reviews.freebsd.org/D16124