BLD/BUG/DOC : made pytz a required dependency

closes #3423
DOC : removed freetype version caveats

minimum version for freetype is 2.3

closes #3412
BLD : updated message when to pytz found
Remove backticks around None. This is not a ref
Fix formatting in docstring for dates
Merge pull request #3521 from jenshnielsen/more_doc_fixes

DOC : More doc fixes
BUG : fix handling of flierprop by boxplot

 - moved logic to parse sym from bxp -> boxplot
 - restored documented behavior with `sym=''`
 - change default value of sym from 'b+' -> None
   - should not be visible from outside as default behavior remains
     the same

closes #3459
Fix marker clipping rectangle.
Extended the nbagg backend to support animation.
DOC : fix main-page tags

<pre> -> <tt> for inline markup.
Merge pull request #3543 from tacaswell/fix_mainpage

DOC : fix main-page tags
Fixed firefox keyboard focus bug with nbagg backend.
Updated nbagg backend to default to a transparent figure patch.
Install texlive and other dependencies when building docs.
Fixed the webagg backend to handle figures with transparency.
PEP8 : whitespace fix
Merge pull request #3559 from jenshnielsen/texlive_travis

TST : Install texlive and other dependencies when building docs.
Merge pull request #3559 from pelson/nbagg_enhancments

ENH : improve nbagg backend
BUG : don't assume label in boxpplot_stat

If the user does not pass in a label, do not assign one.

Fixes #3563
Fixed the differencing of images for the webagg/nbagg backends.