Run tests using tox

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

Some related merge reqtests are: !874 (merged), !1027 (merged), !1037 (merged) and !1051 (merged).

---

• conftest.py: Don't use deprecated get_marker() function

  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 c91784abcdfebef2e1e70612193b67efe7fa016a)

• Add requirements files for install, test and plugin dependencies

  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.

• Add tox.ini from 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`.

• .gitignore: Ingnore tox, pytest, coverage files

• .gitlab-ci.yml: Run tests using tox

  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.

• bst2html.py: Name the second argument to avoid an error

  (cherry picked from commit 1fd38fb74c81d39ca7500d9b7f77c67430ed7852)
  
  Backport of !874.

• .coveragerc: Omit some things which are irrelevant to cover.

  * Omit versioneer's _version.py
  
  * Omit our __main__.py which is used only internally for generating
    documentation
  
  (cherry picked from commit 116c1070c893c4b5bd2505ea39157f1b37946092)

• .coveragerc: Omit .tox/ directory from coverage stats

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