Remove tests/alt-fixtures/twine-1.5.0-py2.py3-none-any.whl

This test file was added in 8b8790b (Add tests to ensure version
parsing does not regress, part of pypa#147) with the intent of testing the
target Python version from wheel file names.

However, the filename is identical to the one of the other text
fixture tests/fixtures/twine-1.5.0-py2.py3-none-any.whl thus it does
not appear to add any additional coverage to the code extracting the
target Python version.

tests/test_wheel.py

@@ -22,17 +22,11 @@
 from twine import exceptions
 from twine import wheel
 
-from . import helpers
 
-
-@pytest.fixture(
-    params=[
-        "fixtures/twine-1.5.0-py2.py3-none-any.whl",
-        "alt-fixtures/twine-1.5.0-py2.py3-none-any.whl",
-    ]
-)
+@pytest.fixture()
 def example_wheel(request):
-    file_name = os.path.join(helpers.TESTS_DIR, request.param)
+    parent = pathlib.Path(__file__).parent
+    file_name = str(parent / "fixtures" / "twine-1.5.0-py2.py3-none-any.whl")
     return wheel.Wheel(file_name)