Retirement roadmap for pytest-openfiles

MNT: Drop Python 3.6

Also update CI matrix.

Co-authored-by: James Davies <[email protected]>

to go with https://github.com/astropy/astropy/pull/14041

MNT: Replace distutils with packaging

[ci skip]

TST: Use Actions, update test matrix

TST: Do not use conda for OSX and Windows

Small infrastructure updates

Enable Travis notifications to slack

Force garbage collection to avoid issues with memory-mapped arrays

Refactor package infrastructure

Only import imp when neccesary to avoid deprecation warning

Use LooseVersion to check for pytest 3.6+

With `StrictVersion` it will faill to run in pytest-dev itself:

> ValueError: invalid version n...

Use tool:pytest instead of pytest

Switch to matching filenames from open_files_ignore with fnmatch to allow glob-like syntax

Usage of only pytest is no longer working on pytest 4.2+

Replace use of deprecated pytest method to make compatible with 4.1

Include tests in sdists

Tests are used by some downstreams to verify the build was done correctly.

Update CI matrix to test against newer versions of Python and pytest

Do not include tests in packaged distribution

Add decorator to skip detection of open files

Add documentation to README for ``open_files_ignore``

Update README

Remove astropy as test dependency

Set up CI on travis and appveyor