Changes in 0.4.18

- Make bindings for magic_[set|get]param optional, and throw NotImplementedError
if they are used but not supported.  Only call setparam() in the constructor if
it's supported. This prevents breakage on CentOS7 which uses an old version of
libmagic.

- Add tests for CentOS 7 & 8

Changes in 0.4.16 and 0.4.17

- add MAGIC_MIME_TYPE constant, use that in preference to MAGIC_MIME internally.
This sets up for a breaking change in a future major version bump where
MAGIC_MIME will change to mathch magic.h.
- add magic.version() function to return library version
- add setparam/getparam to control internal behavior
- increase internal limits with setparam to prevent spurious error on some jpeg files
- various setup.py improvements to declare modern python support
- support MSYS2 magic dlls
- fix warning about using 'is' on an int in python 3.8
- include tests in source distribution

- many test improvements:
-- tox runner support
-- remove deprecated test_suite field from setup.py
-- docker tests that cover all LTS ubuntu versions
-- add test for snapp file identification

- doc improvements
-- document dependency install process for debian
-- various typos
-- document test running process
