make the various manager classes in the notebook app configurable

like NotebookManager already was
test notebook checkpoints in subdirectories
monkeypatch inspect.findsource in a context manager

rather than all the time, so it doesn't affect use outside IPython
log error message when API requests fail
create checkpoints dir in notebook subdirectories
Quick typo fix in nbformat/convert.py
Merge pull request #5732 from jdfreder/quickfix

Quick typo fix in nbformat/convert.py
Merge pull request #5714 from minrk/warn-api-errors

log error message when API requests fail
get rid of redundant context manager
Merge pull request #5708 from minrk/checkpoints-path

create checkpoints dir in notebook subdirectories
Merge pull request #5712 from minrk/inspect.getsource-context

monkeypatch inspect.findsource in a context manager
Merge pull request #5700 from minrk/configurable-managers

make the various manager classes in the notebook app configurable
rev kernel protocol version, and include it in message headers (as a string)
kernel_info versions are strings
pyin -> execute_input
pyout -> execute_result

nbformat and nbconvert are not affected
pyerr -> error
remove user_variables

leave only user_expressions
remove unused channel definitions from managerabc

they were moved to channelsabc, but the originals were never removed.
pass on msgspec doc with updates