text: Title: 03_linear_regression_sol.py can't run with errors as follows
  Body: Traceback (most recent call last):
    File "D:/stanford_tensorflow_tutorials/tf_oreilly/01_linear_regression_sol.py", line 19, in <module>
      book = xlrd.open_workbook(DATA_FILE, encoding_override="utf-8")
    File "C:\Anaconda3\lib\site-packages\xlrd\__init__.py", line 441, in open_workbook
      ragged_rows=ragged_rows,
    File "C:\Anaconda3\lib\site-packages\xlrd\book.py", line 107, in open_workbook_xls
      bk.fake_globals_get_sheet()
    File "C:\Anaconda3\lib\site-packages\xlrd\book.py", line 687, in fake_globals_get_sheet
      self.get_sheets()
    File "C:\Anaconda3\lib\site-packages\xlrd\book.py", line 678, in get_sheets
      self.get_sheet(sheetno)
    File "C:\Anaconda3\lib\site-packages\xlrd\book.py", line 669, in get_sheet
      sh.read(self)
    File "C:\Anaconda3\lib\site-packages\xlrd\sheet.py", line 1475, in read
      self.update_cooked_mag_factors()
    File "C:\Anaconda3\lib\site-packages\xlrd\sheet.py", line 1543, in update_cooked_mag_factors
      elif not (10 <= zoom <= 400):
  TypeError: unorderable types: int() <= NoneType()