Write a Python function canonicalize_date(date_str) that takes a string date_str, representing a date in one of several possible formats, and returns a string that is the canonical representation of that date.

I. Input Representation of Dates

The string passed to the canonicalize_date(date_str) function can be in any of the following three formats.

yyyy-mm-dd, where yyyy is a 4-digit sequence giving the year; mm is a 1- or 2-digit sequence giving the month; and dd is a 1- or 2-digit sequence giving the date.

Example: 2017-02-31

mm/dd/yyyy, where yyyy is a 4-digit sequence giving the year; mm is a 1- or 2-digit sequence giving the month; and dd is a 1- or 2-digit sequence giving the date.

Example: 02/31/2017.

MonthName dd yyyy, where MonthName is a three-letter sequence giving the name of a month (one of: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov,Dec); dd is a 1- or 2-digit sequence giving the date; andyyyy is a 4-digit sequence giving the year.

Example: Feb 31 2017

NOTE: To simplify programming, we will assume that all months have 31 days.

II. Canonical Representation of Dates

A date with year yyyy, month mm, and day dd, where yyyy, mm, and dd are strings of digits, has a canonical (i.e., standard) representation given by

"{:d}-{:d}-{:d}".format( int(yyyy ), int(mm),int(dd))

The reason for converting yyyy, mm, and dd to numbers using int() and then back to strings is to remove any leading zeros.