Installation fails on python 2.7

Hi I'm installing a fresh copy of this plugin over my docker instance and I'm getting:

```
...
Adding ckanext-scheming 2.1.0 to easy-install.pth file

Installed /srv/app/src_extensions/ckanext-scheming
Processing dependencies for ckanext-scheming==2.1.0
Searching for pyyaml
Reading https://pypi.org/simple/pyyaml/
Downloading https://files.pythonhosted.org/packages/36/2b/61d51a2c4f25ef062ae3f74576b01638bebad5e045f747ff12643df63844/PyYAML-6.0.tar.gz#sha256=68fb519c14306fec9720a2a5b45bc9f0c8d1b9c72adf45c37baedfcd949c35a2
Best match: PyYAML 6.0
Processing PyYAML-6.0.tar.gz
Writing /tmp/easy_install-hdquDQ/PyYAML-6.0/setup.cfg
Running PyYAML-6.0/setup.py -q bdist_egg --dist-dir /tmp/easy_install-hdquDQ/PyYAML-6.0/egg-dist-tmp-lli1fC
Traceback (most recent call last):
File "/srv/app/src_extensions/ckanext-scheming/setup.py", line 43, in
""",
File "/usr/lib/python2.7/site-packages/setuptools/__init__.py", line 162, in setup
return distutils.core.setup(**attrs)
File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
dist.run_commands()
File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
self.run_command(cmd)
File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
cmd_obj.run()
File "/usr/lib/python2.7/site-packages/setuptools/command/develop.py", line 38, in run
self.install_for_development()
File "/usr/lib/python2.7/site-packages/setuptools/command/develop.py", line 156, in install_for_development
self.process_distribution(None, self.dist, not self.no_deps)
File "/usr/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 758, in process_distribution
[requirement], self.local_index, self.easy_install
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 782, in resolve
replace_conflicting=replace_conflicting
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1065, in best_match
return self.obtain(req, installer)
File "/usr/lib/python2.7/site-packages/pkg_resources/__init__.py", line 1077, in obtain
return installer(requirement)
File "/usr/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 685, in easy_install
return self.install_item(spec, dist.location, tmpdir, deps)
File "/usr/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 711, in install_item
dists = self.install_eggs(spec, download, tmpdir)
File "/usr/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 896, in install_eggs
return self.build_and_install(setup_script, setup_base)
File "/usr/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 1164, in build_and_install
self.run_setup(setup_script, setup_base, args)
File "/usr/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 1150, in run_setup
run_setup(setup_script, args)
File "/usr/lib/python2.7/site-packages/setuptools/sandbox.py", line 253, in run_setup
raise
File "/usr/lib/python2.7/contextlib.py", line 35, in __exit__
self.gen.throw(type, value, traceback)
File "/usr/lib/python2.7/site-packages/setuptools/sandbox.py", line 195, in setup_context
yield
File "/usr/lib/python2.7/contextlib.py", line 35, in __exit__
self.gen.throw(type, value, traceback)
File "/usr/lib/python2.7/site-packages/setuptools/sandbox.py", line 166, in save_modules
saved_exc.resume()
File "/usr/lib/python2.7/site-packages/setuptools/sandbox.py", line 141, in resume
six.reraise(type, exc, self._tb)
File "/usr/lib/python2.7/site-packages/setuptools/sandbox.py", line 154, in save_modules
yield saved
File "/usr/lib/python2.7/site-packages/setuptools/sandbox.py", line 195, in setup_context
yield
File "/usr/lib/python2.7/site-packages/setuptools/sandbox.py", line 250, in run_setup
_execfile(setup_script, ns)
File "/usr/lib/python2.7/site-packages/setuptools/sandbox.py", line 45, in _execfile
exec(code, globals, locals)
File "/tmp/easy_install-hdquDQ/PyYAML-6.0/setup.py", line 67, in

```

This is due to this:
https://github.com/yaml/pyyaml/issues/476

Probably we can enforce the correct version (5.4) here to fix

https://github.com/ckan/ckanext-scheming/blob/a5b5522302a3cabb94f5bdb1a30a3bb7a5133403/setup.py#L28

Start in setup.py around line 28, where the dependency version is defined, and review the linked PyYAML issue for the Python 2.7 incompatibility. Reproduce the installation failure with the Docker setup, then verify that dependency resolution selects PyYAML 5.4 and the plugin installs successfully.

Written by the indexing model from the issue text.