Restore PyYAML 5.4.1 to avoid TypeError in webassets 0.12.1

[danielbreves]

The patch to bump pyyaml to 6.0.1 introduced in https://github.com/keitaroinc/docker-ckan/pull/108/files#diff-132f494f7fa90591cc8efc8b166716154979ee515aa16ab9fd614f82b92a9f07R64 is raising an error when running the CKAN 2.9 image. CKAN 2.9.9 requires webassets 0.12.1, which calls yaml.load() without a Loader. This generated a warning in pyyaml 5.4.1, but raises a TypeError in 6.x. Unfortunately, there's an issue building PyYAML < 6.0.1 from source because it's not compatible with Cython 3, so this PR follows the workaround suggested in yaml/pyyaml#736 to create a constraint file that limits the Cython version to one that should work and restores pyyaml to 5.4.1.

Traceback (most recent call last):
  File "/usr/bin/ckan", line 8, in <module>
    sys.exit(ckan())
  File "/usr/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python3.8/site-packages/click/core.py", line 781, in main
    with self.make_context(prog_name, args, **extra) as ctx:
  File "/usr/lib/python3.8/site-packages/click/core.py", line 700, in make_context
    self.parse_args(ctx, args)
  File "/srv/app/src/ckan/ckan/cli/cli.py", line 116, in parse_args
    result = super(ExtendableGroup, self).parse_args(ctx, args)
  File "/usr/lib/python3.8/site-packages/click/core.py", line 1212, in parse_args
    rest = Command.parse_args(self, ctx, args)
  File "/usr/lib/python3.8/site-packages/click/core.py", line 1048, in parse_args
    value, args = param.handle_parse_result(ctx, opts, args)
  File "/usr/lib/python3.8/site-packages/click/core.py", line 1630, in handle_parse_result
    value = invoke_param_callback(self.callback, ctx, self, value)
  File "/usr/lib/python3.8/site-packages/click/core.py", line 123, in invoke_param_callback
    return callback(ctx, param, value)
  File "/srv/app/src/ckan/ckan/cli/cli.py", line 126, in _init_ckan_config
    _add_ctx_object(ctx, value)
  File "/srv/app/src/ckan/ckan/cli/cli.py", line 135, in _add_ctx_object
    ctx.obj = CtxObject(path)
  File "/srv/app/src/ckan/ckan/cli/cli.py", line 57, in __init__
    self.app = make_app(self.config)
  File "/srv/app/src/ckan/ckan/config/middleware/__init__.py", line 56, in make_app
    load_environment(conf)
  File "/srv/app/src/ckan/ckan/config/environment.py", line 123, in load_environment
    p.load_all()
  File "/srv/app/src/ckan/ckan/plugins/core.py", line 161, in load_all
    unload_all()
  File "/srv/app/src/ckan/ckan/plugins/core.py", line 208, in unload_all
    unload(*reversed(_PLUGINS))
  File "/srv/app/src/ckan/ckan/plugins/core.py", line 236, in unload
    plugins_update()
  File "/srv/app/src/ckan/ckan/plugins/core.py", line 153, in plugins_update
    environment.update_config()
  File "/srv/app/src/ckan/ckan/config/environment.py", line 172, in update_config
    webassets_init()
  File "/srv/app/src/ckan/ckan/lib/webassets_tools.py", line 67, in webassets_init
    create_library(u'vendor', os.path.join(
  File "/srv/app/src/ckan/ckan/lib/webassets_tools.py", line 28, in create_library
    library = YAMLLoader(config_path).load_bundles()
  File "/usr/lib/python3.8/site-packages/webassets/loaders.py", line 162, in load_bundles
    obj = self.yaml.load(f) or {}
TypeError: load() missing 1 required positional argument: 'Loader'

cc @Filip3mac

[danielbreves]

Fixes keitaroinc/ckan-helm#101

[stojanovskis1]

Looks good.