See https://stackoverflow.com/a/60086137/562769

Closes #1623

pypdf has three types of tests: (1) Tests that work only with the repository itself, (2) tests that require the sample-files git submodule, and (3) tests that require PDF documents from external websites.

As people or projects might not be able to execute (2), they are marked with `@pytest.mark.samples`

As people or projects might not be able to execute (3), they are marked with `@pytest.mark.enable_socket` (formerly @pytest.mark.external).

We regularly had issues with people forgetting to mark their tests with `external` (see #1632).

The [pytest-socket](https://pypi.org/project/pytest-socket/) pytest plugin disables the usage of sockets on the whole project, except for the tests marked with `enable_socket`. This was, we can guarantee that the CI will catch missing `@pytest.mark.enable_socket`.

* "/N" attributes wrongly ignored during copy process
* The object referenced via `src[field]` needs to be a dictionary

Fixes #1615, #1671
Fixes #1673

Robustness (ROB)
-  Some attributes not copied in DictionaryObject._clone (#1635)
-  Allow merging multiple time pages with annots (#1624)

Testing (TST)
-  Replace pytest.mark.external by enable_socket (#1657)

[Full Changelog](3.5.0...3.5.1)