404 and other errors when calling `delete()` method on `Github.Secret.Secret` and `GitHub.Variable.Variable`

Methods called on Secret and Variable model methods are not requesting the correct GitHub REST API endpoint URLs. Invoking these methods causes 404 errors.

• For Variable, there is a bug that appends additional incorrect parts to the variable's REST API endpoint URL, resulting in 404 errors for delete (HTTP DELETE) and edit (HTTP PATCH).
  • Example:
    • Expected: /repos/{owner}/{repo}/actions/variables/{name}
    • Actual: /repos/{owner}/{repo}/actions/variables/{name}/actions/variables/{name}
• For both Variable and Secret, fetching a PaginatedList of either from the Repository model's get_variables or get_secrets methods fails to correctly set the _url attribute on the paginated list of models. Therefore, calling such methods will also fail.

Below are some examples for how to reproduce the unexpected behavior. Please substitute "AndrewJDawes/PyGithub" with the owner/name of a repository to which you have access.

# Setup
repo = self.g.get_repo("AndrewJDawes/PyGithub")
variable = repo.create_variable("VARIABLE_NAME", "VARIABLE_VALUE")
repo = self.g.get_repo("AndrewJDawes/PyGithub")
variable = repo.get_variable("VARIABLE_NAME")


# Issue
variable.delete() # github.GithubException.UnknownObjectException: 404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest"}

repo = self.g.get_repo("AndrewJDawes/PyGithub")
repo.create_variable("VARIABLE_ONE_NAME", "VARIABLE_ONE_VALUE")
repo.create_variable("VARIABLE_TWO_NAME", "VARIABLE_TWO_VALUE")
repo = self.g.get_repo("AndrewJDawes/PyGithub")
variables = repo.get_variables()
for variable in variables:
    if variable.name in ["VARIABLE_ONE_NAME", "VARIABLE_TWO_NAME"] and variable.value in ["VARIABLE_ONE_VALUE", "VARIABLE_TWO_VALUE"]:
        variable.delete() # Fails to delete Variable

  repo = self.g.get_repo("AndrewJDawes/PyGithub")
  repo.create_secret("SECRET_ONE_NAME", "SECRET_ONE_VALUE")
  repo.create_secret("SECRET_TWO_NAME", "SECRET_TWO_VALUE")
  repo = self.g.get_repo("AndrewJDawes/PyGithub")
  secrets = repo.get_secrets()
  for secret in secrets:
      if secret.name in ["SECRET_ONE_NAME", "SECRET_TWO_NAME"]:
          secret.delete() # AttributeError: 'NoneType' object has no attribute 'startswith'

Start with the Secret and Variable model methods and the Repository.get_variables and get_secrets entry points. Reproduce the listed delete and edit failures, then compare the generated REST URLs and paginated model _url values with the expected endpoints. Done means these operations work for individual and paginated Secret and Variable objects.