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.

src/pip/_internal/locations.py

@@ -72,8 +72,10 @@ def get_src_prefix():
     # Use getusersitepackages if this is present, as it ensures that the
     # value is initialised properly.
     user_site = site.getusersitepackages()
+    user_base = site.getuserbase()
 except AttributeError:
     user_site = site.USER_SITE
+    user_base = site.USER_BASE
 
 if WINDOWS:
     bin_py = os.path.join(sys.prefix, 'Scripts')
@@ -84,7 +86,7 @@ def get_src_prefix():
         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')
+    bin_user = os.path.join(user_base, '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