Update to include details about how to re-enable.

Closes #7715

This ensures that on python2 the values passed into `check_output` or
`Popen` are bytes.  If they are passed in as unicode (due to
unicode_literals at the top of most of our files) they will cause an
exception when there is a path in PATH that has non-ascii values.  The
reason is that when locating possible executable our input (as
unicode) is concatenated with the non-ascii path (as bytes) which then
fails to encode as ascii and raises.

The other two places we currently use `check_output` (setupext.py and
tools/github_stats.py) we do not need this handling as we do not
import unicode_iterals in those files.

Without this change, zooming in from ticks separated by
days to ticks separated by multiples of hours leaves tick
labels with no date information, even when the time period
covers several days.

With the change there is a systematic progression from one
level to the next, dropping the largest unit and adding a
smaller unit.

BUG: fix checkout_output with non-ascii paths in PATH

STY: fix poor date-time format defaults

Api doc backport no imag build

As part of scaling dashes with linewidth, unscaled versions of
the dash pattern are now tracked.  Make sure we copy these as well.

To prevent double-scaling of the pattern in the legend

Closes #7814

Give correct name for toolkits_tests parameter to set in setup.cfg.

DOC: fix name of toolkit test in API doc

DOC: matplotlib -> Matplotlib

Fix collection legend handlers