Description:
`
prance validate --backend=openapi-spec-validator swagger.yaml
Traceback (most recent call last):
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 583, in _build_master
ws.require(__requires__)
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 900, in require
needed = self.resolve(parse_requirements(requirements))
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 791, in resolve
raise VersionConflict(dist, req).with_context(dependent_req)
pkg_resources.ContextualVersionConflict: (PyYAML 3.13 (/usr/lib/python3.7/site-packages), Requirement.parse('PyYAML>4,>=4.2b4'), {'prance'})

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/bin/prance", line 6, in <module>
from pkg_resources import load_entry_point
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3191, in <module>
@_call_aside
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3175, in _call_aside
f(*args, **kwargs)
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 3204, in _initialize_master_working_set
working_set = WorkingSet._build_master()
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 585, in _build_master
return cls._build_from_requirements(__requires__)
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 598, in _build_from_requirements
dists = ws.resolve(reqs, Environment())
File "/usr/lib/python3.7/site-packages/pkg_resources/__init__.py", line 786, in resolve
raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'PyYAML>4,>=4.2b4' distribution was not found and is required by prance
`
Same thing happens without specifying validator.

Additional info:
* python-prance 0.14.0-1

Steps to reproduce:
Try to validate any spec file.