Mss

- Windows: ignore missing `SetProcessDPIAware()` on Window XP (fixes 109)
- :heart: contributors: foone

linux.py
- Removed use of `MSS.xlib.XDefaultScreen()`

base.py
- Renamed `MSSBase` class to `MSSMixin`

linux.py
- Renamed `MSS.__del__()` method to `MSS.close()`
- Deleted `MSS.last_error` attribute. Use `LAST_ERROR` constant instead.
- Added `validate()` function
- Added `MSS.get_error_details()` method

windows.py
- Renamed `MSS.__exit__()` method to `MSS.close()`

- MSS: do monitor detection in MSS constructor (fixes 79)
- MSS: specify compliant Python versions for pip install
- tests: enable Python 3.7
- tests: fixed `test_entry_point()` with multiple monitors
- :heart: contributors: hugovk, andreasbuhr

- Linux: fixed a memory leak introduced with 7e8ae5703f0669f40532c2be917df4328bc3985e (fixes 72)
- doc: add the download statistics badge

exception.py
- Added `details` attribute to `ScreenShotError` exception. Empty dict by default.

linux.py
- Added `error_handler()` function