Implement PdfPages for backend pgf

Some trivial py3fications.

The `elif` clauses in `Marker.set_marker` needed to be reordered
because any Python object that has a `__len__` method is an
instance of `collections.Sized`.

remove trailing spaces on a comment line

make seaborn great again on Matplotlib-2.2

Fix for failing bar plot w/ units

(Too many spaces between "layout." and "You", missing betwen "subplots"
and "with".)

Convert NaT to nan in date2num

Fix extra and missing spaces in constrainedlayout warning.

Also reorder other mentioned 3rdparty packages to alphabetical order.

Add mplcairo to 3rdparty docs.

…boilerplate.py

Remove if six.PY2 code paths from boilerplate.py

Support markers from Paths that consist of one line segment

Simplify setupext by using globs.

so don't rely on the 3rd party version.

Also replace py.test by pytest in the docs (recommended since pytest
3.0, https://docs.pytest.org/en/latest/changelog.html#id281; we require
pytest>=3.1 anyways).

Also delete tox.ini which is clearly outdated.

Add a link from the examples to the user's guide.

Clarify that the weakref behavior only affects *methods* used as
callbacks, not free functions.

…strainedlayout

FIX/ENH CL: Allow single parent colorbar w/ gridspec layout

Replace the Locked contextmanager (which locks a directory, preventing
other (cooperative) processes to access it) by a private _lock_path
contextmanager, which locks a single file (or directory).

- The finer grained lock avoids locking out the entire tex cache when
  handling usetex, which is useful when running multiple processes at
  once.

- Python3 implements the `"x"` ("exclusive") mode to open, which we can
  use instead of relying on `makedirs` to achieve a race-free operation
  on the filesystem.

- The previous implementation allowed multiple threads of a single
  process to acquire the same lock, but (for the use cases here, e.g.
  running a tex subprocess) this is actually undesirable.  Removing this
  behavior also simplifies the implementation.

- As far as I can tell, the previous implementation was actually racy:
  in

    retries = 50
    sleeptime = 0.1
    while retries:
        files = glob.glob(self.pattern)
        if files and not files[0].endswith(self.end):
            time.sleep(sleeptime)
            retries -= 1
        else:
            break
    else:
        err_str = _lockstr.format(self.pattern)
        raise self.TimeoutError(err_str)

    # <----- HERE

    if not files:
        try:
            os.makedirs(self.lock_path)
        except OSError:
            pass
    else:  # PID lock already here --- someone else will remove it.
        self.remove = False

  multiple processes can reach "HERE" at the same time and each
  successfully create their own lock.

This ensures that when a cairo-based backend is active, animations also
get saved using backend_cairo, rather than falling back on backend_agg.

Deprecate vestigial Annotation.arrow.

…string

DOC: make legend docstring interpolated

DOC: Update layout of sidebar in documentation

The Agg backend still doesn't know how to make use of that information
in draw_markers, but third-party backends (e.g. mplcairo) do honor the
setting.

Use long color names for default rcParams

…ture

Make function signatures more explicit

Propagate marker antialias setting to GraphicsContext.

Add print_rgba to backend_cairo.

Add some basic smoketesting for webagg (and wx).

- Give the colorbars a reasonable aspect ratio.
- Fix a link.
- Other minor edits.

DOC: Make colorbar tutorial examples look like colorbars.

DOC: Don't use private attribute in tk example.  Fix Toolbar class rename.