Implement P3.9 style dict union (PEP584)#1902

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TheAnyKey wants to merge 19 commits intoRustPython:masterfrom

TheAnyKey:TheAnyKey/p39_dict_union

Commits on Apr 28, 2020

added initial implementation of self defining expressions to fstring (Python3.8)
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Commits on Apr 30, 2020

Commits on May 1, 2020

recommit
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Commits on May 3, 2020

fixed: returned wrong error when union opration is invoked with other type. So far it was a key error now it is a type error.
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reverted toolchain settings to python version 3.8 as 3.9 is so far not supported
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fixed typo
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