Cut new release 3.10

[twiecki]

Release early, release often.

We have a couple of nice new features on the docket and our release process is getting more stream-lined.

We do need a release manager, anyone wants to volunteer?

[twiecki]

Actually with the new MLDA stepper and the switch to Theano-PyMC we should make a 3.10 release.

[twiecki]

We should also reorder the release notes. Put new features at top and MLDA stepper at top. Roughly in order of significance.

[michaelosthege]

We should also reorder the release notes. Put new features at top and MLDA stepper at top. Roughly in order of significance.

When editing the release notes, please link to these PRs that were not yet mentioned:

• fix #4216: str and latex representations for Bound variables #4217

[Spaak]

See #4227 for an update to the release notes mentioning the new string representations.