Make skip-existing work for Artifactory indexes

Artifactory repositories seem to return HTTP 403 when you try to upload
a package that already exists (and your user doesn't have overwrite
permissions).
Extend skip_upload to support this.

tests/test_upload.py

@@ -155,6 +155,20 @@ def test_skip_existing_skips_files_already_on_pypiserver(monkeypatch):
                               package=pkg) is True
 
 
+def test_skip_existing_skips_files_already_on_artifactory(monkeypatch):
+    # Artifactory (https://jfrog.com/artifactory/) responds with 403
+    # when the file already exists.
+    response = pretend.stub(
+        status_code=403,
+        text="Not enough permissions to overwrite artifact "
+             "'pypi-local:twine/1.5.0/twine-1.5.0-py2.py3-none-any.whl'"
+             "(user 'twine-deployer' needs DELETE permission).")
+
+    pkg = package.PackageFile.from_filename(WHEEL_FIXTURE, None)
+    assert upload.skip_upload(response=response,
+                              skip_existing=True,
+                              package=pkg) is True
+
 def test_skip_upload_respects_skip_existing(monkeypatch):
     response = pretend.stub(
         status_code=400,

twine/commands/upload.py

@@ -57,18 +57,22 @@ def skip_upload(response, skip_existing, package):
     filename = package.basefilename
     # NOTE(sigmavirus24): Old PyPI returns the first message while Warehouse
     # returns the latter. This papers over the differences.
-    msg = ('A file named "{0}" already exists for'.format(filename),
+    msg_400 = ('A file named "{0}" already exists for'.format(filename),
            'File already exists')
+    msg_403 = 'Not enough permissions to overwrite artifact'
     # NOTE(sigmavirus24): PyPI presently returns a 400 status code with the
     # error message in the reason attribute. Other implementations return a
-    # 409 status code. We only want to skip an upload if:
+    # 409 or 403 status code. We only want to skip an upload if:
     # 1. The user has told us to skip existing packages (skip_existing is
     #    True) AND
     # 2. a) The response status code is 409 OR
     # 2. b) The response status code is 400 AND it has a reason that matches
-    #       what we expect PyPI to return to us.
+    #       what we expect PyPI to return to us. OR
+    # 2. c) The response status code is 403 AND the text matches what we
+    #       expect Artifactory to return to us.
     return (skip_existing and (response.status_code == 409 or
-            (response.status_code == 400 and response.reason.startswith(msg))))
+            (response.status_code == 400 and response.reason.startswith(msg_400)) or
+            (response.status_code == 403 and msg_403 in response.text)))
 
 
 def upload(dists, repository, sign, identity, username, password, comment,