Update Sympy_printing extension load

Change 

%loaf_ext sympyprinting
to
%load_ext sympy.interactive.ipythonprinting
Merge pull request #3315 from urielshaolin/patch-1

Update sympy_printing extension load

Change 

%loaf_ext sympyprinting
to
%load_ext sympy.interactive.ipythonprinting
Fixed all broken references, refactored some stuff here and there,
standardized nomenclature.   Ready to test...
Merge pull request #3304 from minrk/hwm

set unlimited HWM for all relay devices

libzmq-3 changed the default HWM for sockets to 1000 from infinite, which can result in dropped messages on ROUTER or PUB sockets under high load.

This PR explicitly sets these back to infinite to avoid dropped messages.

 closes #3303
Got RST exporting to work...  Sort of.
Fixed typo as suggested by @damianavila
Renamed pygments file due to namespace polution.
Fixed incorrect rename... whoops
Almost have nbconvert working again...
Fixed problem with data type filter construction.
rename `%%file` to `%%fwrite`

keeps `%%file` as alias

Alternative to #3313, no behavior is changed.
Change hardcoded functions to use reflection instead.
make that writefile
move safe_unicode to py3compat
user_variables and user_expressions use rich repr API

each key will be a rich display_data message rather than plaintext reprs.
test user_variables/expressions in message spec
add user-expressions/variables changes to message spec
Made exporter names match template names.
Moving handlers to subdir
Adding new files.