`requests_cache` is not optional on retry

[toslunar]

Description

requests_cache is unconditionally imported at uncache_url, which is called by fetch_with_retries on retry.

Reproduce

pytest --check-links on a document containing a broken link that returns retry-after header, for example

<a href="https://zenodo.org/badge/latestdoi/11846/arose/nglview">

Stacktrace:

/.../site-packages/_pytest/runner.py:341: in from_call
    result: Optional[TResult] = func()
/.../site-packages/_pytest/runner.py:262: in <lambda>
    lambda: ihook(item=item, **kwds), when=when, reraise=reraise
/.../site-packages/pluggy/_hooks.py:265: in __call__
    return self._hookexec(self.name, self.get_hookimpls(), kwargs, firstresult)
/.../site-packages/pluggy/_manager.py:80: in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
/.../site-packages/pluggy/_callers.py:60: in _multicall
    return outcome.get_result()
/.../site-packages/pluggy/_result.py:60: in get_result
    raise ex[1].with_traceback(ex[2])
/.../site-packages/pluggy/_callers.py:39: in _multicall
    res = hook_impl.function(*args)
/.../site-packages/_pytest/runner.py:177: in pytest_runtest_call
    raise e
/.../site-packages/_pytest/runner.py:169: in pytest_runtest_call
    item.runtest()
/.../site-packages/pytest_check_links/plugin.py:375: in runtest
    response = self.fetch_with_retries(url)
/.../site-packages/pytest_check_links/plugin.py:344: in fetch_with_retries
    self.uncache_url(url_no_anchor)
/.../site-packages/pytest_check_links/plugin.py:353: in uncache_url
    from requests_cache import BaseCache
E   ModuleNotFoundError: No module named 'requests_cache'

Expected behavior

For the broken link, the test failure message should be

FAILED index.html::/.../index.html <a href=https://zenodo.org/badge/latestdoi/11846/arose/nglview>

instead of

FAILED index.html::/.../index.html <a href=https://zenodo.org/badge/latestdoi/11846/arose/nglview> - ModuleNotFoundError: No module named 'requests_cache'

Moreover, temporarily down links could be retried without requests_cache.

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