bump version to 2.7.2 at the beginning of the development cycle #439

Job	Run time
run / test (mysql57, pymysql)	25s
run / test (mysql57, mysqldb)	15s
run / test (mysql57, asyncmy)	9s
run / test (mysql57, mariadbconnector)	16s
run / test (mysql80, pymysql)	22s
run / test (mysql80, mysqldb)	46s
run / test (mysql80, asyncmy)	22s
run / test (mysql80, mariadbconnector)	14s
run / test (mysql84, pymysql)	26s
run / test (mysql84, mysqldb)	31s
run / test (mysql84, asyncmy)	14s
run / test (mysql84, mariadbconnector)	27s
run / test (mysql90, pymysql)	14s
run / test (mysql90, mysqldb)	35s
run / test (mysql90, asyncmy)	33s
run / test (mysql90, mariadbconnector)	27s
run / summarize	1s
	6m 17s

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