Add requirements-dev.txt for pip

Generated with pipenv lock -r -d. Will be used for separating the
development dependencies.

requirements-dev.txt

@@ -0,0 +1,25 @@
+-i https://pypi.org/simple
+atomicwrites==1.2.1
+attrs==18.2.0
+backcall==0.1.0
+decorator==4.3.0
+flake8==3.6.0
+ipython-genutils==0.2.0
+ipython==7.1.1
+jedi==0.13.1
+mccabe==0.6.1
+more-itertools==4.3.0
+parso==0.3.1
+pexpect==4.6.0 ; sys_platform != 'win32'
+pickleshare==0.7.5
+pluggy==0.8.0
+prompt-toolkit==2.0.7
+ptyprocess==0.6.0
+py==1.7.0
+pycodestyle==2.4.0
+pyflakes==2.0.0
+pygments==2.2.0
+pytest==3.10.1
+six==1.11.0
+traitlets==4.3.2
+wcwidth==0.1.7