diff --git a/.travis.yml b/.travis.yml
index 9f8facd15..62c1b223e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -29,7 +29,9 @@ install:
 #  - virtualenv venv
 #  - . venv/bin/activate
   - pip install -r requirements.txt
-  - pip install -r test/requirements.txt
+  # Added --upgrade, because at least pytest already came from some other
+  # install command and so version was never checked
+  - pip install --upgrade -r test/requirements.txt
   - pip install coverage codecov pytest-cov codacy-coverage
   - pip install .
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index d3d04338c..a2dd8a0ab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,9 @@
   `True` and `False` values.
 * Added augmenter `Canny`, which applies canny edge detection with alpha
   blending and random coloring to images.
+* Renamed `imgaug/external/poly_point_isect.py` to `imgaug/external/poly_point_isect_py3.py.bak`.
+  The file is in the library only for completeness and contains python3 syntax.
+  `poly_point_isect_py2py3.py` is actually used.
 
 ## Fixes
  
diff --git a/imgaug/augmenters/meta.py b/imgaug/augmenters/meta.py
index c3dde9967..01cfc50e3 100644
--- a/imgaug/augmenters/meta.py
+++ b/imgaug/augmenters/meta.py
@@ -3255,16 +3255,19 @@ def OneOf(children, name=None, deterministic=False, random_state=None):
 
     Examples
     --------
-    >>> imgs = [np.ones((10, 10))]
+    >>> import imgaug.augmenters as iaa
+    >>> images = [np.ones((10, 10), dtype=np.uint8)]
     >>> seq = iaa.OneOf([
     >>>     iaa.Fliplr(1.0),
     >>>     iaa.Flipud(1.0)
     >>> ])
-    >>> imgs_aug = seq.augment_images(imgs)
+    >>> imgs_aug = seq.augment_images(images)
 
-    flips each image either horizontally or vertically.
+    Flips each image either horizontally or vertically.
 
 
+    >>> import imgaug.augmenters as iaa
+    >>> images = [np.ones((10, 10), dtype=np.uint8)]
     >>> seq = iaa.OneOf([
     >>>     iaa.Fliplr(1.0),
     >>>     iaa.Sequential([
@@ -3274,9 +3277,9 @@ def OneOf(children, name=None, deterministic=False, random_state=None):
     >>>     ]),
     >>>     iaa.Noop()
     >>> ])
-    >>> imgs_aug = seq.augment_images(imgs)
+    >>> imgs_aug = seq.augment_images(images)
 
-    either flips each image horizontally, or adds blur+dropout+noise or does
+    Either flips each image horizontally, or adds blur+dropout+noise or does
     nothing.
 
     """
diff --git a/imgaug/external/poly_point_isect.py b/imgaug/external/poly_point_isect_py3.py.bak
similarity index 98%
rename from imgaug/external/poly_point_isect.py
rename to imgaug/external/poly_point_isect_py3.py.bak
index 7af0a54de..b74c33795 100644
--- a/imgaug/external/poly_point_isect.py
+++ b/imgaug/external/poly_point_isect_py3.py.bak
@@ -1,3 +1,12 @@
+#############################################################################
+# This file uses python3 syntax. The actually used version of this library is
+# in "(...)_py2py3.py". This (original) file is added for completeness.
+# To avoid fatal errors related to the test environment in python2.7, it
+# ends in ".py.bak", so that the file is always ignored.
+# (There are probably more elegant ways to achieve this.)
+#############################################################################
+
+
 # BentleyOttmann sweep-line implementation
 # (for finding all intersections in a set of line segments)
 
diff --git a/test/requirements.txt b/test/requirements.txt
index 7914b45e7..ac19a37a4 100644
--- a/test/requirements.txt
+++ b/test/requirements.txt
@@ -1,4 +1,5 @@
-pytest>=3.0.5
+# 4.6.0 currently causes crashes, see https://github.com/pytest-dev/pytest/issues/5358
+pytest>=3.0.5,<4.6.0
 mock; python_version < '3.3'  # unittest.mock does not exist in older versions
 unittest2; python_version < '3.4'  # in 3.4, self.subTest was added
 xdoctest >= 0.7.2