[issue29312] Use FASTCALL in dict.update()

[Jeroen Demeyer]

Jeroen Demeyer <J.Demeyer at UGent.be> added the comment:

> but it will make d1.update(**d2) slower with a complexity of O(n): d2 must be converted to 2 lists

This part is still true and it causes a slow-down of about 23% for dict.update(**d), see benchmarks at https://github.com/python/cpython/pull/14589#issuecomment-509356084

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<https://bugs.python.org/issue29312>
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