std::array<> caster: support arbitrary sequences (#1602)

This PR brings the std::array<> caster in sync with the other STL type
casters: to accept an arbitrary sequence as input (rather than a list,
which is too restrictive).
diff --git a/tests/test_stl.py b/tests/test_stl.py
index b78f86a..9e58223 100644
--- a/tests/test_stl.py
+++ b/tests/test_stl.py
@@ -216,3 +216,7 @@
     assert len(r) == 1
     del r
     assert cstats.alive() == 0
+
+
+def test_array_cast_sequence():
+    assert m.array_cast_sequence((1, 2, 3)) == [1, 2, 3]