PR #3713 (walrus operator) merged into devel.

[Edward K. Ream]

The devel branch now contains PR #3713.

This branch uses the := operator to convert the pattern:

m = <something, usually an regex pattern>

if m:

    <code>

to:

if m := <something, usually an regex pattern>

    <code>

These changes should be safe:

- I converted the code using regex patterns and have carefully reviewed the diffs.

- The legacy and new code bind m in the same way, so (for example) a following "else" clause will always work as before.

- I have been using the code without problems.  All unit tests pass.

Please report any problems :-)

Edward