Changes to support Python 3

[moorepants]

Fixes #141

• deals with integer division issues
• setup pandas to be < 0.24.0 which supports Python 2 and 3 (as well as
  has get_store and Panel)
• uses items() instead of iteritems() on dictionaries
• deals with range() differences
• fixes issue with random.sample() being more strict
• don't use strptime() as it generates incorrect yaml now
• don't use StandardError
• deal with issue defining a class attribute and a variable not being
  accessible in a list comprehension

Packages I did local testing with:

coverage                  5.5              py37h5e8e339_0    conda-forge
dynamicisttoolkit         0.5.3              pyh9f0ad1d_3    conda-forge
ipython                   7.24.1           py37h085eea5_0    conda-forge
matplotlib                3.4.2            py37h89c1867_0    conda-forge
matplotlib-base           3.4.2            py37hdd32ed1_0    conda-forge
mock                      4.0.3            py37h89c1867_1    conda-forge
nose                      1.3.7                   py_1006    conda-forge
numpy                     1.20.3           py37h038b26d_1    conda-forge
numpydoc                  1.1.0                      py_1    conda-forge
oct2py                    5.2.0              pyh9f0ad1d_0    conda-forge
pandas                    0.23.4          py37h637b7d7_1000    conda-forge
pytables                  3.6.1            py37h0c4f3e0_3    conda-forge
python                    3.7.10          hffdb5ce_100_cpython    conda-forge
pyyaml                    5.4.1            py37h5e8e339_0    conda-forge
scipy                     1.6.3            py37h29e03ee_0    conda-forge
sphinx                    4.0.2              pyh6c4a22f_1    conda-forge
octave              5.2.0