You can specify a range to iterate over with ws.iter_rows():
import openpyxl
wb = openpyxl.load_workbook('C:/workbook.xlsx')
ws = wb['Sheet3']
for row in ws.iter_rows('C{}:C{}'.format(ws.min_row,ws.max_row)):
for cell in row:
print cell.value
Edit: per your comment you want the cell values in a list:
import openpyxl
wb = openpyxl.load_workbook('c:/_twd/2016-06-23_xlrd_xlwt/input.xlsx')
ws = wb.get_sheet_by_name('Sheet1')
mylist = []
for row in ws.iter_rows('A{}:A{}'.format(ws.min_row,ws.max_row)):
for cell in row:
mylist.append(cell.value)
print mylist