Remove import of cached_property from pip

And also remove hack to make it available for python3.7.

Pipenv does not support py37 for quite a while now.

pipenv/resolver.py

@@ -3,11 +3,7 @@
 import logging
 import os
 import sys
-
-try:
-    from functools import cached_property
-except ImportError:
-    cached_property = property
+from functools import cached_property
 
 
 def _ensure_modules():

pipenv/utils/resolver.py

@@ -5,7 +5,7 @@
 import sys
 import tempfile
 import warnings
-from functools import lru_cache
+from functools import cached_property, lru_cache
 from pathlib import Path
 from typing import Dict, List, Optional
 
@@ -31,13 +31,6 @@
 from pipenv.utils.requirements import normalize_name
 from pipenv.vendor import click
 
-try:
-    # this is only in Python3.8 and later
-    from functools import cached_property
-except ImportError:
-    # eventually distlib will remove cached property when they drop Python3.7
-    from pipenv.patched.pip._vendor.distlib.util import cached_property
-
 from .dependencies import (
     HackedPythonVersion,
     convert_deps_to_pip,