Python Mss Versions Save

:heart: contributors: @narumishi

• :snake: produce wheels for Python 3 only
• MSS: renamed again MSSMixin to MSSBase, now derived from abc.ABCMeta
• tools: force write of file when saving a PNG file
• tests: fix tests on macOS with Retina display
• Windows: fixed multi-thread safety (fixes #150)

:heart: contributors: @hugovk, @foone, @SergeyKalutsky

• :warning: removed support for Python 2.7
• MSS: improve type annotations and add CI check
• MSS: use __slots__ for better performances
• MSS: better handle resources to prevent leaks
• MSS: improve monitors finding
• Windows: use our own instances of GDI32 and User32 DLLs
• doc: add project_urls to setup.cfg
• doc: add an example using the multiprocessing module (closes #82)
• tests: added regression tests for #128 and #135
• tests: move tests files into the package

:warning: The source code is now derived from master, where Python 2 support has been dropped.

• new contributors: @grintor, @SergeyKalutsky
• Windows: use our own instances of GDI32 and User32 DLLs (backported from 5.0.0)
• Windows: fix the screen bounding AiO rectangle (backported from 5.0.0)

• new contributor: @foone
• Windows: ignore missing SetProcessDPIAware() on Window XP

• Linux: fix several XLib functions signature (fixes #92)
• Linux: improve monitors finding by a factor of 44
• Linux: fix pylint error "Using variable 'validate' before assignment"
• tests: removed PyPy from the testing matrix
• tests: fixed Travis-CI with Xvfb
• tests: do not allow any failure

• 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

• new contributors: @hugovk, @andreasbuhr
• MSS: do monitor detection in MSS constructor (fixes #79)
• MSS: specify compliant Python versions for pip install
• tests: enable Python 3.7
• tests: fix test_entry_point() with multiple monitors

• Linux: fix a memory leak introduced with 7e8ae5703f0669f40532c2be917df4328bc3985e (fixes #72)
• doc: add the download statistics badge

• Linux: add an error handler for the XServer to prevent interpreter crash (fix #61)
• MSS: fix a ResourceWarning: unclosed file in setup.py
• tests: fix a ResourceWarning: unclosed file
• doc: fix a typo in Screenshot.pixel() method (thanks to @mchlnix)
• big code clean-up using black

• new contributor: @ryanfox
• Windows: enable Hi-DPI awareness