Allow backslashes in pep701 f-strings #277

Job	Run time
Create sdist and Python 3 Wheel	39s
Create Python 2 Wheel	47s
test_typing (sdist)	34s
Test Documentation	35s
test_typing (wheel)	30s
Test Package (2.7, sdist)	15s
Test Package (2.7, wheel)	15s
Test Package (3.3, sdist)	27s
Test Package (3.3, wheel)	23s
Test Package (3.4, sdist)	21s
Test Package (3.4, wheel)	18s
Test Package (3.5, sdist)	18s
Test Package (3.5, wheel)	16s
Test Package (3.6, sdist)	16s
Test Package (3.6, wheel)	20s
Test Package (3.7, sdist)	20s
Test Package (3.7, wheel)	19s
Test Package (3.8, sdist)	15s
Test Package (3.8, wheel)	20s
Test Package (3.9, sdist)	24s
Test Package (3.9, wheel)	23s
Test Package (3.10, sdist)	28s
Test Package (3.10, wheel)	31s
Test Package (3.11, sdist)	26s
Test Package (3.11, wheel)	23s
Test Package (3.12, sdist)	32s
Test Package (3.12, wheel)	30s
	11m 5s

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