Fix structlog by downgrading it

[ericholscher]

Refs #10990

[humitos]

When changing a dependency, make sure you are running pip-compile over all the files. Otherwise, there are some incompatibilities between them.

I usually run,

pip-compile -U requirements/pip.in
for file in requirements/*.in; do pip-compile --output-file=`echo $file | cut -f 1 -d .`.txt $file; done

To make sure that all the files are properly updated.

See #11010