4.0.0

• MSS: remove use of setup.py for setup.cfg
• MSS: renamed MSSBase to MSSMixin in base.py
• MSS: refactor ctypes argtype, restype and errcheck setup (fixes #84)
• Linux: ensure resources are freed in grab()
• Windows: avoid unecessary class attributes
• MSS: ensure calls without context manager will not leak resources or document them (fixes #72 and #85)
• MSS: fix Flake8 C408: Unnecessary dict call - rewrite as a literal, in exceptions.py
• MSS: fix Flake8 I100: Import statements are in the wrong order
• MSS: fix Flake8 I201: Missing newline before sections or imports
• MSS: fix PyLint bad-super-call: Bad first argument 'Exception' given to super()
• tests: use tox, enable PyPy and PyPy3, add macOS and Windows CI