Remove pipenv from setup_requires (#121)

pipenv should not be required for sdist installations.

noticed this behavior when reviewing NixOS/nixpkgs#95689

Author: jonringer

setup.py

@@ -40,7 +40,7 @@ def read_version(package):
     long_description_content_type="text/x-rst",
     packages=find_packages(exclude=exclude_packages),
     install_requires=install_requires,
-    setup_requires=["pipenv"],
+    setup_requires=[],
     extras_require={
         "speedups": [
             'xxhash;platform_python_implementation=="CPython"',