Add a test for csv.register_dialect with both Dialect and kwargs

[sobolevn]

Feature or enhancement

Python docs state:

csv.register_dialect(name[, dialect[, **fmtparams]])

    Associate dialect with name. name must be a string. The dialect can be specified either by passing a sub-class of [Dialect](https://docs.python.org/3/library/csv.html#csv.Dialect), or by fmtparams keyword arguments, or both, with keyword arguments overriding parameters of the dialect. For full details about dialects and formatting parameters, see section [Dialects and Formatting Parameters](https://docs.python.org/3/library/csv.html#csv-fmt-params).

Notice this part:

or both, with keyword arguments overriding parameters of the dialect

But, this is not ever tested in test_csv.
Link:

cpython/Lib/test/test_csv.py

Lines 432 to 449 in f235178

	def test_registry(self):
	class myexceltsv(csv.excel):
	delimiter = "\t"
	name = "myexceltsv"
	expected_dialects = csv.list_dialects() + [name]
	expected_dialects.sort()
	csv.register_dialect(name, myexceltsv)
	self.addCleanup(csv.unregister_dialect, name)
	self.assertEqual(csv.get_dialect(name).delimiter, '\t')
	got_dialects = sorted(csv.list_dialects())
	self.assertEqual(expected_dialects, got_dialects)
	def test_register_kwargs(self):
	name = 'fedcba'
	csv.register_dialect(name, delimiter=';')
	self.addCleanup(csv.unregister_dialect, name)
	self.assertEqual(csv.get_dialect(name).delimiter, ';')
	self.assertEqual([['X', 'Y', 'Z']], list(csv.reader(['X;Y;Z'], name)))

Pitch

Let's add a test that ensures that register_dialect do work as documented.

[JelleZijlstra]

PRs were merged, thank you!