Merge pull request #4258 from minrk/backport-1.2

to-backport label is now 1.2
Some small fixes and changes of nb version conversion
Use plain string searching for test exclusions.

Closes gh-4243
Make check for pywin32 more portable.

The attribute is message in Python 2, msg in Python 3.
Merge pull request #4153 from Carreau/timeitobj

make timeit return an object that contains values
Merge pull request #4151 from takluyver/drop-alias

Refactor alias machinery
Keywords should shadow magic functions
Disable autocall for prefilter test
Removing page.less from base style.less.
Merge pull request #4229 from jdfreder/clearoutput

Clear_output: Animation & widget related changes.
Review changes
Merge pull request #4259 from takluyver/win-test-exclusions

Fix Windows test exclusions
Unify exit status from system_raw and system_piped
Add test for exit status from system_piped
Fixing a formatting error in the custom display example notebook.
Merge pull request #4261 from ngoldbaum/master

Fixing a formatting error in the custom display example notebook.
Small fix
more informative error message for bad serialization
check that the unpacked test object is correct
test invalid Session pack and unpack