In Express, set session to None when importing shared

[wch]

This is one possible solution for #1079.

When it sees import shared or from shared import ... at the top level, it puts that within a with session_context(None): block.

One possible limitation is that, if someone does this:

import module1, shared

Then that gets transformed into

with session_context(None):
    import module1, shared

which means that module1 would also have the session set to None. In practice, it seems unusual for people to put multiple imports on a line like that, but still, it could happen.