dazl v7.5

[da-tanabe]

dazl v7.5 is intended to contain a preview release of dazl v8's API while still retaining the current API in its entirety.

I had originally intended for dazl v7.5 the release to be a bridge between 7 and 8, but in initially trying to implement 8, the differences between the two seemed too far to bridge. So implementation split between master (v8) and release-7.3 (v7.3.7–v7.4.0).

• Unrelated to the new v8 API, I was trying to troubleshoot mypy errors on internal projects. This forced me to get the release branch passing mypy's typechecker 100%.
• The code on master, however, does not pass mypy's typechecker, and trying to get that to happen has actually uncovered a few bugs in the implementation of the v8 API.
• I started down the path of backporting "safe" v8 classes to the v7 release branch (python: Backport some new v8 classes to v7 #203), but in attempting this, it's now actually clear that the original plan of a v7.5 release that safely introduces the new API while leaving the existing v7 code largely alone is actually attainable, and that this release can happen off of master.

The plan:

• Restore master's compatibility with Python 3.6
• Restore removed symbols (notably dazl.model.types and dazl.model.types_store) on master
• Merge release-7.3 into master, retaining its bug fixes as identified by the typechecker
• Fix the remaining type checking errors (most of this work has already been done on python: Backport some new v8 classes to v7 #203; it's just tough to review because the base branch is not master)
• Final testing and release