Those were deprecations implemented in 0.15
* all middlewares have been moved from `werkzeug.wsgi` to
`werkzeug.middleware`, including the `SharedDataMiddleware` we use
* ProxyFix was moved to werkzeug.middleware.proxy_fix, this had
already been fixed but I forgot the import
* sessions support was moved to a separate package
(`pallets/secure-cookies`), however while distros are starting to
update werkzeug to 1.0 (e.g. done on Arch, and in Debian
Experimental) they're not bundling secure-cookies so using a
vendored version seems like the least bad thing we can do, even more
so as conditional dependencies are not really a thing (e.g. even
with just pip we can't depend on secure-cookie iff werkzeug >= 1.0)