bump version to 2.7.2 at the beginning of the development cycle #440

Job	Run time
run / test (mysql57, pymysql)	2h 41m 49s
run / test (mysql57, mysqldb)	3h 6m 33s
run / test (mysql57, asyncmy)	3h 9m 46s
run / test (mysql57, mariadbconnector)	3h 7m 2s
run / test (mysql80, pymysql)	2h 35m 6s
run / test (mysql80, mysqldb)	2h 39m 29s
run / test (mysql80, asyncmy)	2h 41m 32s
run / test (mysql80, mariadbconnector)	2h 40m 18s
run / test (mysql84, pymysql)	1h 11m 52s
run / test (mysql84, mysqldb)	2h 32m 0s
run / test (mysql84, asyncmy)	2h 32m 30s
run / test (mysql84, mariadbconnector)	2h 32m 27s
run / test (mysql90, pymysql)	1s
run / test (mysql90, mysqldb)	1s
run / test (mysql90, asyncmy)	1h 15m 36s
run / test (mysql90, mariadbconnector)	2h 28m 4s
run / summarize	1s
	1d 11h 14m 7s

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