Merge rust-bitcoin/rust-bitcoincore-rpc#348: Use rust-bitcoin-maintai… #385

Job	Run time
Test - 1.56.1 toolchain (minimal)	3m 0s
Test - stable toolchain (minimal)	40s
Test - 1.56.1 toolchain (recent)	2m 50s
Integration tests - stable toolchain (0.18.0)	52s
Test - stable toolchain (recent)	43s
Test - nightly toolchain (minimal)	40s
Integration tests - stable toolchain (0.18.1)	51s
Test - nightly toolchain (recent)	42s
Integration tests - stable toolchain (0.19.0.1)	56s
Integration tests - stable toolchain (0.19.1)	1m 10s
Integration tests - stable toolchain (0.20.0)	59s
Integration tests - stable toolchain (0.20.1)	59s
Integration tests - stable toolchain (0.21.0)	1m 2s
	15m 24s

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