Add dependabot to auto update github actions #53

Job	Run time
doctests	55s
test (ubuntu-latest, x64, 1.6)	1m 26s
test (ubuntu-latest, x64, nightly)	2m 14s
test (ubuntu-latest, x86, 1.6)	1m 27s
test (ubuntu-latest, x86, nightly)	2m 16s
test (macOS-latest, x64, 1.6)	1m 59s
test (macOS-latest, x64, nightly)	5m 37s
test (windows-latest, x64, 1.6)	9m 11s
test (windows-latest, x64, nightly)	8m 24s
test (windows-latest, x86, 1.6)	8m 0s
test (windows-latest, x86, nightly)	8m 27s
	49m 56s

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