v6.19.08 Remove Python2 related code and support

• fix #105 by converting empty string to None, which has smtplib handling
• fix #226 by handling all Exception instead of just OSError on SIGCHLD to parent
• fix #239 by catching ssl.SSLError
• Merge pull request #237 from meeuw/integration-tests
• remove python 2 code: only supporting python 3
• Merge pull request #238 from yshingai/v6.19.07-add-alias
• support windows-874 cp874
• Merge pull request #234 from meeuw/fix-incorrect-fsf-address
• Merge pull request #233 from meeuw/fix-license
• #220 allow uid_cache=true (latest UID from oldmail-file) in addition to uid_cache=