Add python version requirement.

[nostachio]

Currently one can install this using pip install -e . in a Python 2 environment, which succeeds. However, trying to run the program in a Python 2 environment via python -m workout examples/easy.yaml results in errors:

$ python -m workout examples/easy.yaml 

Traceback (most recent call last):
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 163, in _run_module_as_main
    mod_name, _Error)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 111, in _get_module_details
    __import__(mod_name)  # Do not catch exceptions initializing package
  File "workout/__init__.py", line 5, in <module>
    from .workouts import workout
  File "workout/workouts.py", line 26
    display(f'\nrunning workout: "{self.name}" with sections:')

This change creates the requirement to use python 3 or better during pip install step, reducing confusion for those who have lower versions of python as the default (e.g. all OSX users) due to errors after a successful install. With the changes, if the pip install -e . command is run, error output is produced letting the user know Python >=3 is required:

$ pip install -e .
[...]
Obtaining file:///Users/xxxx/python_projects/Script_My_Workout
ERROR: Package 'workout' requires a different Python: 2.7.16 not in '>=3'

[twhughes]

thanks for the PR!