Bump version to 2.3.0
Use appnope in qt and wx gui support from the terminal; closes #6189

On Mavericks, the qt and wx gui support was unusable from the
ipython console in a terminal.  Using nope() in enable_qt4 and
enable_wx, and reversing with nap() in disable_qt4 and disable_wx,
solves the problem.
Merge pull request #6321 from efiring/fix_nap_qt_wx

Use appnope in qt and wx gui support from the terminal; closes #6189
Backport PR #6333: allow slash in root->tree redirect

- before #6117, only `/base_url/` worked
- after #6117, only `/base_url` worked
- after this PR, both should work.

closes #6329
Backport PR #6395: update mathjax CDN url in nbconvert template

closes #6392
Backport PR #6410: Fix relative import in appnope

Closes gh-6409

Ping @minrk - this fix should go into appnope as well.
Backport PR #6374: fix _abort_queues typo
Backport PR #6318: use write_error instead of get_error_html

get_error_html is removed in tornado 4.0

closes #6312

should be backported to 2.x, since it's a compatibility issue.
Backport PR #6303: Fix error message when failing to load a notebook

- fix missing import in nbformat.reader.reads
- show JSON error message if there is one

Before, opening a v4 notebook would show:

...
Backport PR #6556: Use some more informative asserts in inprocess kernel tests

Prompted by gh-6552 (but doesn't fix it)
Backport PR #6483: Trim anchor link in heading cells, fixes #6324

This should fix #6324, which was really irritating me :smiley: I used `.trim()` as suggested by @minrk.
fixes #6473
Merge pull request #6585 from Carreau/fixes-6473

fixes #6473
Backport PR #6269: Implement atomic save

this should avoid issues with corrupted/lost notebooks when the disk is full
Backport PR #6453: Copy file metadata in atomic save

Closes gh-6405
Backport PR #6581: Properly mock winreg functions for test

Fixes a lot of test failures on Python 3.4 on Windows, caused by importlib trying to use our mocked out winreg functions when reload(path) was called.
Backport PR #6583: Windows symlink test fixes

This should fix the Windows 3.x test failure we've been seeing recently, and another related failure I found when trying to reproduce it on my Windows VM.
Backport PR #6480: only compare host:port in Websocket.check_origin

don't include protocol

- matches tornado's own behavior
- avoids incorrect 403 in http-behind-https proxy situations.

...
Backport PR #6514: Fix for copying metadata flags

Only became apparent when tests ran on a BSD based system.
Backport PR #6587 : Support %matplotlib qt5 and %matplotlib nbagg