Split requirements-dev.txt from requirements.txt

All pytest and types requirements are moved.
Also, the pip bug comment is made more explanatory.

requirements-dev.txt

@@ -0,0 +1,7 @@
+-r requirements.txt
+pytest==5.2.2
+pytest-asyncio==0.10.0
+pytest-mock==1.10.3
+pytest-mypy==0.4.1
+pytest-flakes==4.0.0
+types-certifi==2020.4.0

requirements.txt

@@ -1,11 +1,7 @@
 -e .
-pytest==5.2.2
-pytest-asyncio==0.10.0
-pytest-mock==1.10.3
-pytest-mypy==0.4.1
-pytest-flakes==4.0.0
-# because of pip bug https://github.com/pypa/pip/issues/4780
+# Copied from setup.py because of a pip bug
+#   see https://github.com/pypa/pip/issues/4780
 aiohttp==3.5.4
 certifi==2019.3.9
-types-certifi==2020.4.0
 psutil==5.6.6; sys_platform == 'darwin'
+# End of copy from setup.py