Warning related to soft_unicode

[rsyring]

DeprecationWarning: 'soft_unicode' has been renamed to 'soft_str'. The old name will be removed in MarkupSafe 2.1.

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/rsyring/projects/keg-src/keg/tests/test_view_templating.py", line 11, in test_templating
    resp = self.testapp.get('/template1')
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/webtest/app.py", line 327, in get
    return self.do_request(req, status=status,
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask_webtest.py", line 224, in do_request
    response = super(TestApp, self).do_request(*args, **kwargs)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/webtest/app.py", line 623, in do_request
    res = req.get_response(app, catch_exc_info=True)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/webob/request.py", line 1309, in send
    status, headers, app_iter, exc_info = self.call_application(
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/webob/request.py", line 1278, in call_application
    app_iter = application(self.environ, start_response)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/webtest/lint.py", line 201, in lint_app
    iterator = application(environ, start_response_wrapper)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/werkzeug/local.py", line 375, in <lambda>
    __call__ = lambda x, *a, **kw: x._get_current_object()(*a, **kw)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/app.py", line 2464, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/app.py", line 2450, in wsgi_app
    response = self.handle_exception(e)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/app.py", line 1867, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/views.py", line 89, in view
    return self.dispatch_request(*args, **kwargs)
  File "/home/rsyring/projects/keg-src/keg/web.py", line 159, in dispatch_request
    response = self.render()
  File "/home/rsyring/projects/keg-src/keg/web.py", line 216, in render
    return flask.render_template(self.calc_template_name(), **self.template_args)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/templating.py", line 137, in render_template
    return _render(
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/flask/templating.py", line 120, in _render
    rv = template.render(context)
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/jinja2/environment.py", line 1090, in render
    self.environment.handle_exception()
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/jinja2/environment.py", line 832, in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/jinja2/_compat.py", line 28, in reraise
    raise value.with_traceback(tb)
  File "/home/rsyring/projects/keg-src/keg_apps/web/templates/templating/template1.html", line 7, in top-level template code
    {{ _('Hello %(name)s!') | format(name=name) }}
  File "/home/rsyring/projects/keg-src/.tox/py39-lowest/lib/python3.9/site-packages/jinja2/filters.py", line 823, in do_format
    return soft_unicode(value) % (kwargs or args)
SystemError: <built-in function soft_unicode> returned a result with an error set

Newer versions of Jinja clean this up, but the above comes from our "lowest" tests which use Flask<2.0 and related. Can clean this up when we stop supporting Flask 1.x.