Fix uninstallation of user scripts

User scripts are installed to ~/.local/bin. pip was looking for scripts
to uninstall in ~/.local/lib/python3.8/site-packages/bin. This commit
makes it look in ~/.local/bin instead.

news/8733.bugfix

@@ -0,0 +1 @@
+Correctly uninstall scripts installed with --user

src/pip/_internal/locations.py

@@ -75,16 +75,24 @@ def get_src_prefix():
 except AttributeError:
     user_site = site.USER_SITE
 
+def _get_bin_user():
+    scheme = "{}_user".format(os.name)
+    if scheme not in sysconfig.get_scheme_names():
+        scheme = "posix_user"  # Default to POSIX for unknown platforms.
+    path = sysconfig.get_path("scripts", scheme=scheme)
+    assert path is not None
+    return path
+
+bin_user = _get_bin_user()
+
 if WINDOWS:
     bin_py = os.path.join(sys.prefix, 'Scripts')
-    bin_user = os.path.join(user_site, 'Scripts')
     # buildout uses 'bin' on Windows too?
     if not os.path.exists(bin_py):
         bin_py = os.path.join(sys.prefix, 'bin')
         bin_user = os.path.join(user_site, 'bin')
 else:
     bin_py = os.path.join(sys.prefix, 'bin')
-    bin_user = os.path.join(user_site, 'bin')
 
     # Forcing to use /usr/local/bin for standard macOS framework installs
     # Also log to ~/Library/Logs/ for use with the Console.app log viewer

src/pip/_internal/req/req_uninstall.py

@@ -46,6 +46,7 @@ def _script_names(dist, script_name, is_gui):
         bin_dir = bin_user
     else:
         bin_dir = bin_py
+    assert bin_dir is not None
     exe_name = os.path.join(bin_dir, script_name)
     paths_to_remove = [exe_name]
     if WINDOWS:
@@ -562,6 +563,7 @@ def from_dist(cls, dist):
                     bin_dir = bin_user
                 else:
                     bin_dir = bin_py
+                assert bin_dir is not None
                 paths_to_remove.add(os.path.join(bin_dir, script))
                 if WINDOWS:
                     paths_to_remove.add(os.path.join(bin_dir, script) + '.bat')