Expose version function in package root

[heitorlessa]

With Python 3.6 being dropped in v2, we can use use importlib to expose a version function.

Since importlib looks up DIST files to gather package information, it'll impact cold start performance. Instead, we should create a version and use lazy import to amortize the overhead.

NOTE: importlib.metadata is only available in 3.8. Reducing priority and assigning a help wanted.