Update from 1.12.0 to 1.13.0.

<ChangeLog>

*) Feature: basic support for HTTP reverse proxying.

*) Feature: compatibility with Python 3.8.

*) Bugfix: memory leak in Python application processes when the close

handler was used.

*) Bugfix: threads in Python applications might not work correctly.

*) Bugfix: Ruby on Rails applications might not work on Ruby 2.6.

*) Bugfix: backtraces for uncaught exceptions in Python 3 might be

logged with significant delays.

*) Bugfix: explicit setting a namespaces isolation option to false might

have enabled it.

</ChangeLog>