input: Write a python function to generate and return a calendar for a given month and year.
  assert generate_calendar(2020, 4) == '''     April 2020
  Mo Tu We Th Fr Sa Su
         1  2  3  4  5
   6  7  8  9 10 11 12
  13 14 15 16 17 18 19
  20 21 22 23 24 25 26
  27 28 29 30
  '''
  assert generate_calendar(2021, 2) == '''   February 2021
  Mo Tu We Th Fr Sa Su
   1  2  3  4  5  6  7
   8  9 10 11 12 13 14
  15 16 17 18 19 20 21
  22 23 24 25 26 27 28
  '''
output: import calendar
  
  def generate_calendar(year, month):
      cal = calendar.TextCalendar(firstweekday=0)
      return cal.formatmonth(year, month)