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 console scripts installed to ~/.local/bin

src/pip/_internal/locations.py

@@ -75,16 +75,17 @@ def get_src_prefix():
 except AttributeError:
     user_site = site.USER_SITE
 
+bin_user = sysconfig.get_path('scripts')
+assert bin_user is not None
+
 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