pipenv: restrict pytest<5.0.0

This prevents a test exception that presents like:

    tests/transform_csl.py:46:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    fatcat_tools/transforms/csl.py:204: in citeproc_csl
        style_path = get_style_filepath(style)
    .venv/lib/python3.5/site-packages/citeproc_styles/__init__.py:74: in get_style_filepath
        if resource_exists(__name__, independent_style):
    .venv/lib/python3.5/site-packages/pkg_resources/__init__.py:1134: in resource_exists
        return get_provider(package_or_requirement).has_resource(resource_name)
    .venv/lib/python3.5/site-packages/pkg_resources/__init__.py:1404: in has_resource
        return self._has(self._fn(self.module_path, resource_name))
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    self = <pkg_resources.NullProvider object at 0x7f4f38c0bb00>
    path = '/home/bnewbold/code/fatcat/python/.venv/lib/python3.5/site-packages/citeproc_styles/styles/bibtex.csl'

        def _has(self, path):
            raise NotImplementedError(
    >           "Can't perform this operation for unregistered loader type"
            )
    E       NotImplementedError: Can't perform this operation for unregistered loader type

1 files changed, 1 insertions, 1 deletions

diff --git a/python/Pipfile b/python/Pipfile
index 35465d2b..dbc42557 100644
--- a/python/Pipfile
+++ b/python/Pipfile
@@ -8,7 +8,7 @@ verify_ssl = true
 name = "pypi"
 
 [dev-packages]
-pytest = ">=4"
+pytest = ">=4,<5.0.0"
 pytest-pythonpath = "*"
 pytest-pylint = "*"
 ipython = "*"