Switch CI build to use Python 3.7

[desilinguist]

xlrd has dropped support for XLSX files starting with v2.0. Therefore, when using an XLSX file as input, the current RSMTool package will raise the following error:

Traceback (most recent call last):
  File "/private/scratch/nmadnani/rsmtool/rsmdev/bin/rsmtool", line 33, in <module>
    sys.exit(load_entry_point('rsmtool', 'console_scripts', 'rsmtool')())
  File "/private/scratch/nmadnani/rsmtool/rsmtool/rsmtool.py", line 372, in main
    run_experiment(abspath(args.config_file),
  File "/private/scratch/nmadnani/rsmtool/rsmtool/rsmtool.py", line 145, in run_experiment
    data_container = reader.read()
  File "/private/scratch/nmadnani/rsmtool/rsmtool/reader.py", line 370, in read
    dataframe = self.read_from_file(set_path, converter, **kwargs)
  File "/private/scratch/nmadnani/rsmtool/rsmtool/reader.py", line 263, in read_from_file
    df = do_read(filename, **kwargs)
  File "/private/scratch/nmadnani/rsmtool/rsmdev/lib/python3.8/site-packages/pandas/util/_decorators.py", line 296, in wrapper
    return func(*args, **kwargs)
  File "/private/scratch/nmadnani/rsmtool/rsmdev/lib/python3.8/site-packages/pandas/io/excel/_base.py", line 304, in read_excel         sh %s'
    io = ExcelFile(io, engine=engine)
  File "/private/scratch/nmadnani/rsmtool/rsmdev/lib/python3.8/site-packages/pandas/io/excel/_base.py", line 867, in __init__
    self._reader = self._engines[engine](self._io)
  File "/private/scratch/nmadnani/rsmtool/rsmdev/lib/python3.8/site-packages/pandas/io/excel/_xlrd.py", line 22, in __init__
    super().__init__(filepath_or_buffer)
  File "/private/scratch/nmadnani/rsmtool/rsmdev/lib/python3.8/site-packages/pandas/io/excel/_base.py", line 353, in __init__
    self.book = self.load_workbook(filepath_or_buffer)
  File "/private/scratch/nmadnani/rsmtool/rsmdev/lib/python3.8/site-packages/pandas/io/excel/_xlrd.py", line 37, in load_workbook
    return open_workbook(filepath_or_buffer)
  File "/private/scratch/nmadnani/rsmtool/rsmdev/lib/python3.8/site-packages/xlrd/__init__.py", line 170, in open_workbook
    raise XLRDError(FILE_FORMAT_DESCRIPTIONS[file_format]+'; not supported')
xlrd.biffh.XLRDError: Excel xlsx file; not supported

[desilinguist]

It looks like pandas.read_excel() should use openpyxl automatically if it is installed and needed.

[desilinguist]

The reason that the Travis CI build failed is because conda-forge currently has no conda package for pandas=1.2.0 -- the version that automatically uses openpyxl for XLSX files instead of xlrd -- for Python 3.6.

$> conda search -c conda-forge pandas=1.2.0
Loading channels: done
# Name                       Version           Build  Channel             
pandas                         1.2.0  py37hdc94413_0  conda-forge         
pandas                         1.2.0  py38h51da96c_0  conda-forge         
pandas                         1.2.0  py39hde0f152_0  conda-forge

This is because pandas 1.2.0 explicitly dropped support for Python 3.6.

It might be time to do the same for RSMTool going forward. For now, the workaround for Python 3.6 is to install the older version of xlrd:

conda install xlrd=1.2.0

[desilinguist]

Done in #498 and #499.