Do not overlap python_version requirements

This was noticed while trying to install the package using poetry and encountering an issue [like this](python-poetry/poetry#1238)

Author: Code0x58

setup.py

@@ -34,7 +34,7 @@ def read(fname):
         'pytest>=2.8,<4.7; python_version<"3.5"',
         'pytest>=2.8; python_version>="3.5"',
         'mypy>=0.570,<0.700; python_version<"3.5"',
-        'mypy>=0.570; python_version>="3.5"',
+        'mypy>=0.570; python_version>="3.5" and python_version<"3.8.0b1"',
         'mypy>=0.701; python_version>="3.8.0b1"',
     ],
     classifiers=[