Simplified test structure a bit, so we no longer need to set PYTHONPATH

The u'' string prefix was reintroduced in Python 3.3, and since we've
dropped py3.2 support we can avoid the whole py{2k,3k}constants.py mess.
diff --git a/rsa/_compat.py b/rsa/_compat.py
index c654770..6fb9b62 100644
--- a/rsa/_compat.py
+++ b/rsa/_compat.py
@@ -44,11 +44,9 @@
 try:
     # < Python3
     unicode_type = unicode
-    have_python3 = False
 except NameError:
     # Python3.
     unicode_type = str
-    have_python3 = True
 
 # Fake byte literals.
 if str is unicode_type:
diff --git a/tests/constants.py b/tests/constants.py
deleted file mode 100644
index 9a96774..0000000
--- a/tests/constants.py
+++ /dev/null
@@ -1,22 +0,0 @@
-# -*- coding: utf-8 -*-
-#
-#  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
-#
-#  Licensed under the Apache License, Version 2.0 (the "License");
-#  you may not use this file except in compliance with the License.
-#  You may obtain a copy of the License at
-#
-#      http://www.apache.org/licenses/LICENSE-2.0
-#
-#  Unless required by applicable law or agreed to in writing, software
-#  distributed under the License is distributed on an "AS IS" BASIS,
-#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#  See the License for the specific language governing permissions and
-#  limitations under the License.
-
-from rsa._compat import have_python3
-
-if have_python3:
-    from py3kconstants import *
-else:
-    from py2kconstants import *
diff --git a/tests/py2kconstants.py b/tests/py2kconstants.py
deleted file mode 100644
index 0b53a78..0000000
--- a/tests/py2kconstants.py
+++ /dev/null
@@ -1,17 +0,0 @@
-# -*- coding: utf-8 -*-
-#
-#  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
-#
-#  Licensed under the Apache License, Version 2.0 (the "License");
-#  you may not use this file except in compliance with the License.
-#  You may obtain a copy of the License at
-#
-#      http://www.apache.org/licenses/LICENSE-2.0
-#
-#  Unless required by applicable law or agreed to in writing, software
-#  distributed under the License is distributed on an "AS IS" BASIS,
-#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#  See the License for the specific language governing permissions and
-#  limitations under the License.
-
-unicode_string = u"Euro=\u20ac ABCDEFGHIJKLMNOPQRSTUVWXYZ"
diff --git a/tests/py3kconstants.py b/tests/py3kconstants.py
deleted file mode 100644
index c12a39b..0000000
--- a/tests/py3kconstants.py
+++ /dev/null
@@ -1,17 +0,0 @@
-# -*- coding: utf-8 -*-
-#
-#  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
-#
-#  Licensed under the Apache License, Version 2.0 (the "License");
-#  you may not use this file except in compliance with the License.
-#  You may obtain a copy of the License at
-#
-#      http://www.apache.org/licenses/LICENSE-2.0
-#
-#  Unless required by applicable law or agreed to in writing, software
-#  distributed under the License is distributed on an "AS IS" BASIS,
-#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#  See the License for the specific language governing permissions and
-#  limitations under the License.
-
-unicode_string = "Euro=\u20ac ABCDEFGHIJKLMNOPQRSTUVWXYZ"
diff --git a/tests/test_strings.py b/tests/test_strings.py
index 541b317..2b9a1d1 100644
--- a/tests/test_strings.py
+++ b/tests/test_strings.py
@@ -22,7 +22,7 @@
 
 import rsa
 
-from constants import unicode_string
+unicode_string = u"Euro=\u20ac ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 
 
 class StringTest(unittest.TestCase):
diff --git a/tox.ini b/tox.ini
index 3844f22..7c8c1c1 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,8 +6,6 @@
 addopts = -v --cov rsa --cov-report term-missing
 
 [testenv]
-setenv =
-    PYTHONPATH = {toxinidir}:{toxinidir}/tests
 commands=py.test []
 deps=pyasn1 >=0.1.3
      coverage >=3.5