Tests are not picked up for pytest >=3.7 on LTS

For some yet unknown reason the tests are not picked up by the test collection only the rst files from the narrative documentation on the LTS branch when using pytest 3.7+

I think at least understanding, preferable fixing, this is critical for the release.

================================================= test session starts ==================================================
platform darwin -- Python 3.7.0, pytest-3.7.4, py-1.6.0, pluggy-0.7.1

Running tests with Astropy version 2.0.10.dev19739.
Running tests in astropy docs.

Date: 2018-11-24T13:44:00

Platform: Darwin-17.7.0-x86_64-i386-64bit

Executable: /usr/local/opt/python/bin/python3.7

Full Python Version: 
3.7.0 (default, Sep 18 2018, 18:47:22) 
[Clang 9.1.0 (clang-902.0.39.2)]

encodings: sys: utf-8, locale: UTF-8, filesystem: utf-8
byteorder: little
float info: dig: 15, mant_dig: 15

Numpy: 1.15.4
Scipy: 1.1.0
Matplotlib: 3.0.2
h5py: 2.8.0
Pandas: 0.23.4
Cython: 0.28.5
astropy_helpers: 3.1rc1
Using Astropy options: remote_data: none.

rootdir: /private/var/folders/dc/hsm7tqpx2d57n7vb3k1l81xw0000gq/T/astropy-test-l155hvv3/lib/python3.7/site-packages, inifile: setup.cfg
plugins: cov-2.6.0, arraydiff-0.2
collected 168 items                                                                                                    

docs/changelog.rst .                                                                                             [  0%]
docs/credits.rst .                                                                                               [  1%]
docs/getting_started.rst .                                                                                       [  1%]
docs/importing_astropy.rst .                                                                                     [  2%]
...
...
docs/whatsnew/2.0.rst .                                                                                          [ 99%]
docs/whatsnew/index.rst .                                                                                        [100%]

============================================== 168 passed in 8.68 seconds ==============================================

cc @dan @eteq @astrofrog

[astrofrog]

This is actually fixed in pytest 4.0. It looks like the commit that broke things was https://github.com/pytest-dev/pytest/commit/2b1410895e9ac30c4f08e9a701017cce65a9717b (in https://github.com/pytest-dev/pytest/pull/3389) and the one that fixed it was https://github.com/pytest-dev/pytest/commit/f8b944dee0b59bdc483ce505738ea1cb3a57a5b4 (in https://github.com/pytest-dev/pytest/pull/4319). I think it’s ok to simply say that we don’t support pytest >=3.7,<4 on LTS since it’s a real pytest bug.

[drdavella]

Also @bsipocz just wanted to mention that using the pytest command is not and will not be supported on LTS, so it’s not surprising that it did not work for you.