www/py-boto3: Update to 1.12.39

Update to 1.12.39 (not the latest versions 1.12.41) to match
devel/py-botocore, updated in ports r531560 which caused a regression
due to mismatched version-spec (>=1.13.6,<1.14.0).

PR: 245709
Reported by: Henrik Hudson <ports thelanman net>, pkg-fallout