Improve pytest configuration to allow specific tests as CLI args

The previous test configuration made it difficult to run a single test
with the pytest CLI. There were two major issues:

- The Tests directory was not a package. It now includes a __init__.py
  file and imports from other tests modules are done with relative
  imports.

- setup.cfg always specified the Tests directory. So even if a specific
  test were specified as a CLI arg, this configuration would also always
  include all tests. This configuration has been removed to allow
  specifying a single test on the command line.

Contributors can now run specific tests with a single command such as:

  $ tox -e py37 -- Tests/test_file_pdf.py::TestFilePdf.test_rgb

This makes it easy and faster to iterate on a single test failure and is
very familiar to those that have previously used tox and pytest.

When running tox or pytest with no arguments, they still discover and
runs all tests in the Tests directory.

Author: jdufresne

Tests/__init__.py

@@ -1,4 +1,4 @@
-from helper import unittest, PillowTestCase, hopper
+from .helper import unittest, PillowTestCase, hopper
 
 # Not running this test by default. No DOS against Travis CI.
 

Tests/bench_cffi_access.py

@@ -1,4 +1,4 @@
-import helper
+from . import helper
 import timeit
 
 import sys

Tests/bench_get.py

@@ -1,4 +1,4 @@
-from helper import unittest, PillowTestCase
+from .helper import unittest, PillowTestCase
 from PIL import Image
 
 TEST_FILE = "Tests/images/fli_overflow.fli"

Tests/check_fli_overflow.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python
 
 from __future__ import division
-from helper import unittest, PillowTestCase
+from .helper import unittest, PillowTestCase
 import sys
 from PIL import Image
 

Tests/check_imaging_leaks.py

@@ -1,4 +1,4 @@
-from helper import unittest, PillowTestCase
+from .helper import unittest, PillowTestCase
 import sys
 from PIL import Image
 from io import BytesIO

Tests/check_j2k_leaks.py

@@ -1,5 +1,5 @@
 from PIL import Image
-from helper import unittest, PillowTestCase
+from .helper import unittest, PillowTestCase
 
 
 class TestJ2kEncodeOverflow(PillowTestCase):

Tests/check_j2k_overflow.py

@@ -1,4 +1,4 @@
-from helper import unittest, PillowTestCase, hopper
+from .helper import unittest, PillowTestCase, hopper
 from io import BytesIO
 import sys
 

Tests/check_jpeg_leaks.py

@@ -1,6 +1,6 @@
 import sys
 
-from helper import unittest, PillowTestCase
+from .helper import unittest, PillowTestCase
 
 # This test is not run automatically.
 #

Tests/check_large_memory.py

@@ -1,6 +1,6 @@
 import sys
 
-from helper import unittest, PillowTestCase
+from .helper import unittest, PillowTestCase
 
 # This test is not run automatically.
 #