Build

bump version to 0.3.0-alpha.6 #6

Job	Run time
build_all_features (linux)	2m 19s
build_all_features (windows)	6m 19s
build_all_features (macos)	1m 57s
build_feature_electrum (linux)	2m 12s
build_feature_esplora (linux)	2m 18s
build_feature_electrum (windows)	6m 59s
build_no_default_features (linux)	1m 55s
build_feature_esplora (windows)	6m 12s
build_no_default_features (windows)	5m 49s
build_feature_electrum (macos)	2m 42s
build_feature_esplora (macos)	2m 20s
build_no_default_features (macos)	1m 52s
	42m 54s

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