mail/exmh2: require tk 8.7 to avoid no such variable error
tk 8.6 produces the following error while 8.7 works:
can't read "state(tag)": no such variable
while executing
"string compare $value $state(tag)"
(procedure "TextUndoTagAddProc" line 10)
invoked from within
"TextUndoTagAddProc state .msgframe.msg.t tagoff link 2200.35"MFH: 2021Q1