Update dev-requirements.txt: require pytest 3; use default bumpversion

we need pytest 3 for some tests use `pytest.approx()`.

we no longer need patched bumpversion, as we now call `git tag` ourselves (in `python setup.py release`)

dev-requirements.txt

@@ -1,7 +1,3 @@
-pytest>=2.8
-virtualenv>=15.0
-tox>=2.3
-
-# We use a fork of bumpversion due to missing support for annotated tags.
-# See peritus/bumpversion#58
-git+https://github.com/anthrotype/bumpversion.git@annotated-tags#egg=bumpversion
+pytest>=3.0
+tox>=2.5
+bumpversion>=0.5.3