[issue2551019] roundup.cgi.actions.ExportCSVAction broken for python3

New submission from Tom Ekberg:

Problem: exporting to a CSV file fails when using python3

Repeat by:
Install roundup using python3.
  * Bring up a tracker,
  * click 'home' unless already there,
  * click 'Download as CSV'

Error description:
  TypeError: byte string value expected, value of type str found

Location of failure:
  write method of the Writer class in roundup/cgi/wsgi_handler.py

Cause of error:
  Python3 expects the argument to the write method to be a byte string
(e.g. b'somestring') where python 2.7 expects a string.

Solution:
Change the write method to convert the string using the encode function
for utf-8. In python 2.7 this encode function returns a string which is
== to the argument. In python 3 it returns a byte string. The attached
hg diff file contains the details.

----------
components: Infrastructure, Web interface
files: wsgi_handler.py.hgdiff
messages: 6325
nosy: tekberg
severity: normal
status: new
title: roundup.cgi.actions.ExportCSVAction broken for python3
versions: devel

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<https://issues.roundup-tracker.org/issue2551019>
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