pipenv --clear is not compatible with pip 9
Be sure to check the existing issues (both open and closed!).

------------

##### Issue description

`do_clean` imports `pip._internal`, available only in 10+.

##### Expected result

Cleans the caches.

##### Actual result

```
$ pipenv --clear
Clearing caches…
Traceback (most recent call last):
  File "/Users/uranusjr/.local/bin/pipenv", line 11, in <module>
    load_entry_point('pipenv', 'console_scripts', 'pipenv')()
  File "/Users/uranusjr/Documents/programming/python/pipenv/pipenv/pipenv/vendor/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/Users/uranusjr/Documents/programming/python/pipenv/pipenv/pipenv/vendor/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/Users/uranusjr/Documents/programming/python/pipenv/pipenv/pipenv/vendor/click/core.py", line 1043, in invoke
    return Command.invoke(self, ctx)
  File "/Users/uranusjr/Documents/programming/python/pipenv/pipenv/pipenv/vendor/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/uranusjr/Documents/programming/python/pipenv/pipenv/pipenv/vendor/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/Users/uranusjr/Documents/programming/python/pipenv/pipenv/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/uranusjr/Documents/programming/python/pipenv/pipenv/pipenv/cli.py", line 225, in cli
    do_clear()
  File "/Users/uranusjr/Documents/programming/python/pipenv/pipenv/pipenv/core.py", line 132, in do_clear
    import pip._internal.locations
ModuleNotFoundError: No module named 'pip._internal'
```

diff --git a/pipenv/core.py b/pipenv/core.py
index 361942e0..8c21468c 100644
--- a/pipenv/core.py
+++ b/pipenv/core.py
@@ -129,13 +129,20 @@ project = Project(which=which)
 
 def do_clear():
     click.echo(crayons.white("Clearing caches…", bold=True))
-    import pip._internal.locations
+    try:
+        from pip._internal import locations
+    except ImportError:     # pip 9.
+        from pip import locations
 
     try:
         shutil.rmtree(PIPENV_CACHE_DIR)
-        shutil.rmtree(pip._internal.locations.USER_CACHE_DIR)
-    except FileNotFoundError:
-        pass
+        shutil.rmtree(locations.USER_CACHE_DIR)
+    except OSError as e:
+        # Ignore FileNotFoundError. This is needed for Python 2.7.
+        import errno
+        if e.errno == errno.ENOENT:
+            pass
+        raise
 
 
 def load_dot_env():
['pipenv/core.py']