Add dependabot support for GitHub Actions #40

Job	Run time
Build ijson and run unit tests (ubuntu-20.04, 3.7)	33s
Build ijson and run unit tests (ubuntu-20.04, 3.8)	30s
Build ijson and run unit tests (ubuntu-20.04, 3.9)	22s
Build ijson and run unit tests (ubuntu-20.04, 3.10)	15s
Build ijson and run unit tests (ubuntu-20.04, 3.11)	17s
Build ijson and run unit tests (ubuntu-20.04, 3.12)	39s
Build ijson and run unit tests (windows-2019, 3.7)	1m 27s
Build ijson and run unit tests (windows-2019, 3.8)	56s
Build ijson and run unit tests (windows-2019, 3.9)	39s
Build ijson and run unit tests (windows-2019, 3.10)	1m 7s
Build ijson and run unit tests (windows-2019, 3.11)	1m 34s
Build ijson and run unit tests (windows-2019, 3.12)	39s
Build ijson and run unit tests (macos-12, 3.7)	31s
Build ijson and run unit tests (macos-12, 3.8)	23s
Build ijson and run unit tests (macos-12, 3.9)	25s
Build ijson and run unit tests (macos-12, 3.10)	29s
Build ijson and run unit tests (macos-12, 3.11)	32s
Build ijson and run unit tests (macos-12, 3.12)	47s
Build ijson and run unit tests (ubuntu-20.04, pypy3.9)	37s
Build ijson and run unit tests (ubuntu-20.04, pypy3.10)	28s
	13m 10s

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