fix(web): issue2551356. Add etag header for not-modified (304) request. When a 304 is returned to a conditional request for a static file, print an ETag for the response. ETag was always sent with a 200 response. This also adds initial support for if-none-match conditional requests for static files. Changes: Refactors the if-modified-since code out to a method. It moves a file stat call from serve_static_file to _serve_file so that an etag can be generated by both serve_static_file and serve_file which call _serve_file. Tests added. This does not test the codepath where serve_file pulls content from the database rather than from a local file on disk. Test mocking _serve_file changed to account for 5th argument to serve_file BREAKING CHANGE: function signature for client.py-Client::_serve_file() now has 5 not 4 parameters (added etag param). Since this is a "hidden" method I am not too worried about it.

# In Python 3 the "anydbm" module was renamed to be "dbm" which is now a
# package containing the various implementations. The "wichdb" module's
# whichdb() function was moved to the new "dbm" module.

try:
    # old school first because <3 had a "dbm" module too...
    import anydbm
    from whichdb import whichdb
except ImportError:
    # python 3+
    import dbm as anydbm
    whichdb = anydbm.whichdb