Starting from `pytest` version 4.1.0, `Node.get_marker()` has been
removed, and hence our tests break when running with newer versions of
`pytest`. It was deprecated since a while back but it has recently been
removed completely. Use `get_closest_marker()` as a replacement that is
suggested in the changelog, and seems to work fine for our use case.

See https://github.com/pytest-dev/pytest/pull/4564 for more context on
the upstream issue.

One way of verifying this change is that this should fix the recently
added `tests-fedora-update-deps` job, that was failing before due to
this issue.

(cherry picked from commit c91784ab)

Add `.in` and `.txt` requirements files for BuildStream's pure python
dependencies. For each pair, the `.in` file is supposed to capture the
loose version requirements, and the corresponding `.txt` file is
supposed to have frozen requirements. We have 3 such sets:

* `requirements`: BuildStream's runtime dependencies
* `dev-requirements`: Dependencies for running tests
* `plugin-requirements`: Dependencies for core plugins

Note that the frozen requirements files will only be used for testing
purposes, and `setup.py` will continue to read loose requirements.

This commit comes with a Makefile to update the requirements files as
well.

---

!1027 is related MR for the master branch.

`tox.ini` has seen heavy churn lately, and backporting individual
commits is turning out to be a more involved process as the two branches
have diverged a bit. So, this is more or less copy-pasted from the
master branch, without a separate lint job.

Since switching to `pycodestyle` will require some code formatting
changes, that does not seem reasonable to apply to this branch. So,
let's stick with running the linters via `pytest`.

This commit a backport of a handful of merge requests to master. As
these two branches have diverged a bit and the master branch has seen a
fair bit of churn lately, it was not trivial to simply cherry-pick the
changes.

Some related merge reqtests are: !1027, !1037 and !1051.

Jim MacArthur authored Oct 16, 2018 and Chandan Singh committed Jan 28, 2019

(cherry picked from commit 1fd38fb7)

Backport of !874.

Tristan Van Berkom authored Jan 09, 2019 and Chandan Singh committed Jan 28, 2019

  * Omit versioneer's _version.py

  * Omit our __main__.py which is used only internally for generating
    documentation

(cherry picked from commit 116c1070)

This is relevant bit of change from the MR !1051, to fix the coverage
stats.

Run tests using tox

See merge request !1111