Build: bugfix RepositoryError.CLONE_ERROR message

This commit converts the `@property` used to select the correct message
depending on the platform (.org or .com) by a `@classmethod` to make it simpler
to realize that we need to call it as a regular method.

In Python 3.9, we will be able to combine these two decorators and we won't need
parenthesis anymore. For now, we need to add parenthesis to make the call of the
method.

Author: humitos

readthedocs/projects/exceptions.py

@@ -51,11 +51,16 @@ class RepositoryError(BuildUserError):
         "Please check the command output for more information.",
     )
 
-    @property
-    def CLONE_ERROR(self):  # noqa: N802
+    # NOTE: we are not using `@property` here because Python 3.8 does not
+    # suport `@property` together with `@classmethod`. On Python >= 3.9, we
+    # could call `RepositoryError.CLONE_ERROR` without parenthesis and it will
+    # work. However, for now, we are just using a class method and calling it
+    # as a function/method.
+    @classmethod
+    def CLONE_ERROR(cls):  # noqa: N802
         if settings.ALLOW_PRIVATE_REPOS:
-            return self.PRIVATE_ALLOWED
-        return self.PRIVATE_NOT_ALLOWED
+            return cls.PRIVATE_ALLOWED
+        return cls.PRIVATE_NOT_ALLOWED
 
     def get_default_message(self):
         return self.GENERIC_ERROR

readthedocs/vcs_support/backends/bzr.py

@@ -38,7 +38,7 @@ def clone(self):
         try:
             self.run("bzr", "checkout", self.repo_url, ".")
         except RepositoryError:
-            raise RepositoryError(RepositoryError.CLONE_ERROR)
+            raise RepositoryError(RepositoryError.CLONE_ERROR())
 
     @property
     def tags(self):

readthedocs/vcs_support/backends/git.py

@@ -200,7 +200,7 @@ def clone(self):
             code, stdout, stderr = self.run(*cmd)
             return code, stdout, stderr
         except RepositoryError:
-            raise RepositoryError(RepositoryError.CLONE_ERROR)
+            raise RepositoryError(RepositoryError.CLONE_ERROR())
 
     @property
     def lsremote(self):

readthedocs/vcs_support/backends/hg.py

@@ -43,7 +43,7 @@ def clone(self):
             )
             return output
         except RepositoryError:
-            raise RepositoryError(RepositoryError.CLONE_ERROR)
+            raise RepositoryError(RepositoryError.CLONE_ERROR())
 
     @property
     def branches(self):

readthedocs/vcs_support/backends/svn.py

@@ -62,7 +62,7 @@ def co(self, identifier=None):
             url = self.repo_url
         retcode, out, err = self.run('svn', 'checkout', url, '.')
         if retcode != 0:
-            raise RepositoryError(RepositoryError.CLONE_ERROR)
+            raise RepositoryError(RepositoryError.CLONE_ERROR())
         return retcode, out, err
 
     @property