Removed version pinning from requirements-dev.txt + added mypy

The development should be done with recent versions, and if any issues
arise those should be fixed.

requirements-dev.txt

@@ -1,20 +1,7 @@
 # Primary requirements
 -e .
-pytest==3.4.1
-pytest-cov==2.5.1
-radon==2.2.0  # for the 'radon cc' command
-pyprof2calltree==1.4.3  # for converting profiler output to KCacheGrind input
-
-# Secondary requirements
-attrs==17.4.0
-colorama==0.3.9
-coverage==4.5.1
-flake8==3.5.0
-flake8-polyfill==1.0.2
-mando==0.6.4
-mccabe==0.6.1
-pluggy==0.6.0
-py==1.5.2
-pycodestyle==2.3.1
-pyflakes==1.6.0
-six==1.11.0
+mypy
+pytest
+pytest-cov
+radon  # for the 'radon cc' command
+pyprof2calltree  # for converting profiler output to KCacheGrind input