Renamed the bundled pytest plugin from pytest_datatest to

_pytest_datatest.

With the release of Pytest 3.6.0, pluggy--a pytest dependency
that was recently updated--has been raising the following
error:

  pluggy.PluginValidationError: unknown hook 'pytest_datatest'
  in plugin <module 'datatest._pytest_plugin'

The related behavior was changed in Pytest 3.6.0 in response
to a deprecation in pluggy (pytest-dev/pytest#3487). While
it seems clear that pluggy supported this without problems
(see pytest-dev/pluggy#91), I'm not sure if the latest Pytest
behavior is intentional or a regression. But in either case,
we want to make sure the above error is avoided.

Author: shawnbrown

datatest/_pytest_plugin/__init__.py

@@ -4,14 +4,14 @@
 try:
     # Check if the external development version is installed.
     import pytest_datatest as _
-    from .pytest_datatest import version
-    from .pytest_datatest import version_info
+    from ._pytest_datatest import version
+    from ._pytest_datatest import version_info
 except ImportError:
     try:
         # If there's no external plugin, load the bundled version.
-        from .pytest_datatest import pytest_runtest_makereport
-        from .pytest_datatest import version
-        from .pytest_datatest import version_info
+        from ._pytest_datatest import pytest_runtest_makereport
+        from ._pytest_datatest import version
+        from ._pytest_datatest import version_info
     except ImportError:
         # If there's no Pytest support at all, set dummy version numbers.
         version = '0.0.0'