Signed-off-by: ossdev <ossdev@puresoftware.com>

* Use xenial dist for travis builds
* Use openjdk8 for all travis tests
* Update python build matrix -- add 3.7, drop 3.5/3.6 (keep 2.7, 3.4, pypy2.7)

We have many deprecation warnings in the travis logs for things that are
fixed in newer versions of `pylint` or `pylint`'s dependencies.

Note that `pylint` >= 2.0 does not support python 2, so this will result
in different versions of pylint running for python 2 vs python 3.
Personally, I am just fine with this.

This commits adds new pytest fixtures in prepation for the
migration of unittest.TestCases to pytest test cases. The handling
of temporary dir creation was also changed so that we can use
the pytest tmpdir fixture after the migration.

* Use hardware accelerated CRC32C function if available

* Add doc notice of optional `crc32c` package

No longer necessary since pytest-catchlog has been merged into pytest's core.
This commit addresses the following warning in pytest output:

  pytest-catchlog plugin has been merged into the core, please remove it from your requirements.

Fixes #1379