Deprecate git+git@ form of VCS url

Author: sbidoul

docs/html/reference/pip_install.rst

@@ -409,7 +409,6 @@ Here are the supported forms::
     [-e] git+ssh://git.example.com/MyProject#egg=MyProject
     [-e] git+git://git.example.com/MyProject#egg=MyProject
     [-e] git+file:///home/user/projects/MyProject#egg=MyProject
-    -e git+git@git.example.com:MyProject#egg=MyProject
 
 Passing a branch name, a commit hash, a tag name or a git ref is possible like so::
 

src/pip/_internal/req/req_install.py

@@ -30,6 +30,7 @@
 from pip._internal.operations.install.wheel import install_wheel
 from pip._internal.pyproject import load_pyproject_toml, make_pyproject_path
 from pip._internal.req.req_uninstall import UninstallPathSet
+from pip._internal.utils.deprecation import deprecated
 from pip._internal.utils.hashes import Hashes
 from pip._internal.utils.logging import indent_log
 from pip._internal.utils.marker_files import (
@@ -633,6 +634,18 @@ def update_editable(self, obtain=True):
         vc_type, url = self.link.url.split('+', 1)
         vcs_backend = vcs.get_backend(vc_type)
         if vcs_backend:
+            if not self.link.is_vcs:
+                reason = (
+                    "This form of VCS URL is being deprecated: {}."
+                ).format(
+                    self.link.url
+                )
+                replacement = None
+                if self.link.url.startswith("git+git@"):
+                    replacement = (
+                        "git+git:// or git+ssh://"
+                    )
+                deprecated(reason, replacement, gone_in="21.0")
             hidden_url = hide_url(self.link.url)
             if obtain:
                 vcs_backend.obtain(self.source_dir, url=hidden_url)