Accept label keyword arguments in remove()

[Thanatos9404]

Fixes #820

@csmarchbanks, as discussed in the issue: labels() takes the label values positionally or as keyword arguments, but remove() only took them positionally, so a child created with c.labels(method='get', endpoint='/') had to be removed with c.remove('get', '/'), in labelnames order.

remove() now takes keyword arguments with the same rules as labels():

c = Counter('my_requests_total', 'HTTP Failures', ['method', 'endpoint'])
c.labels(method='post', endpoint='/submit').inc()
c.remove(method='post', endpoint='/submit')

• every label name must be given, and the values are matched by name, not position;
• positional and keyword values can't be mixed (ValueError, same message as labels());
• unknown or missing names raise ValueError.

This is different from remove_by_labels(), which removes every labelset that partially matches a dict (so leaving a label out widens the match) and scans all children. remove() with keywords still deletes exactly one labelset and checks that every label name is given, like labels().

I also moved the remove() docstring to the top of the function: it sat after the first statements, so it wasn't the function's docstring. The labels docs now show the keyword form.

Tests in tests/test_core.py: removal by keyword, matching by name when the keyword order differs from labelnames, the ValueError cases (mixed, missing, unknown and extra names), str() coercion (remove(l=None)), and the keyword form in test_recreate_child_after_removal. The 5 new or extended tests fail on master with TypeError: ... got an unexpected keyword argument.

Run locally: the full pytest suite on Linux with Python 3.12 (431 passed, 14 skipped), tox -e mypy's command (mypy 0.991 on Python 3.9: no issues), and the flake8 and isort --check commands from tox.ini.