new release

[jku]

Last release was more than 4 months ago, should make another one.

There's a couple of large ngclient changesets still in flight:

• urllib changes More porting from requests to urllib3 #2773, Handle proxy variables #2789
• root bootstrap feature Cache all root metadata versions #2767

I think the changes are so significant that we make it v6.0.0 even if theoretically the API might remain 100% compatible.