Use walrus operator for env-assignment syntax

[Technologicat]

Now that Python 3.8 has added the walrus operator :=, it would make more sense to override that (instead of left-shift <<) to indicate env-assignment in let, do, and similar constructs. It would also help make IDEs happy, because := is a name-binding operator also in standard Python.

The rules would remain the same they are now (as of v0.14.3 and v0.15.0): when a name := value operation is encountered, if the context allows an env-assignment, then it is an env-assignment; otherwise leave it unprocessed, delegating to the standard meaning of := (just like we now, somewhat less sensibly, delegate to the standard meaning of <<).

Doing this will remove the need for noqa: F821 for names declared in let and do expressions, as well as allow the Lispy dialect (variant of Lispython) to work without triggering any undefined-name errors in static code analyzers for standard Python, such as flake8.

We could add name := value now as an alternative syntax, and deprecate the old syntax name << value once Python 3.8 becomes the minimum supported version. See #1 for the language version support schedule.

Note to self: To get started on this, grep the codebase for envassign and LShift. These are already handled centrally, so the required changes should be rather minor. We have a version-sensitive wrapper for NamedExpr in unpythonic.syntax.astcompat that will simply not match anything in 3.6 and 3.7.

[Technologicat]

Completed in 83dd632.

Did not touch any noqa comments.

Minimum Python version is now 3.8, since 3.6 and 3.7 are already EOL.

The new syntax is now preferred. However, old syntax will remain available at least for the 0.15.x series.