BUG : fix list comprehensions over class members

Due to scoping fixes in py3k, list comprehensions over class level
attributes during class definition does not work (see
http://stackoverflow.com/questions/13905741/accessing-class-variables-from-a-list-comprehension-in-the-class-definition).

Superficially Fixes #3436. There seem to be other issues
BUG : fix glaring syntax error
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
BUG : fix encoding of png data

When formatting the png data to send over the wire need to decode the
byte string to ascii.  If this is not done the literal string sent
to the browser is:
   "data:image/png;base64,b'iVBOR...'"
instead of
   "data:image/png;base64,iVBOR..."

The extra b' makes the string no longer a valid png which is why
we were getting white boxes
Allow running from non-ascii directory on Windows
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
Clarify matplotlib/PyCXX incompatibility
Merge pull request #3492 from mdboom/clarify-pycxx

Allow python 3 version of PyCXX
BUG: Take the absolute difference between times when determining the correct tick interval
BUG : fix eps corruption when using clipping

_get_clip_path was returning '<built-in function id>' instead
of the ps-function name like it should have been.  This was introduced
in 9b9c0c6fbdd53ec42e2a1edecc9c6895b6f7a3ef in PR #2927.

Closes #3523
Merge pull request #3526 from tacaswell/fix_eps_clip_paths

BUG : fix eps corruption when using clipping
Only insert links to pdfs if we are actually generating these. This should
silence most warnings in the html only small build on travis
Merge pull request #3487 from mdboom/use-getcwdu

BUG : use getcwdu to deal with unicode in user home-path in python 2
Merge pull request #3530 from jenshnielsen/travis_docs

BUG : Only insert links to pdfs if we are actually generating these.
Merge pull request #3510 from blink1073/fix-setupext

BLD : improve setupext.py on windows
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