Python fork

1. Fixed test framework setup
2. Added useful assertion message
3. Modified description
   • included the exception messages to return in the case of invalid parameter(s) (all three languages Python/JS/Ruby return the exact same error messages 'Invalid gender' / 'Invalid weight' / 'Invalid duration')
   • fixed formatting
   • removed minor typo

The description should specify in which order the exceptions are evaluated and what error messages should be used. In this case, gender is evaluated first, then weight, then duration.

In the technical details section/paragraph there is a typo for the word formula. It is spelled "formulae" in the kata

Exception messages aren't detailed.

Ruby :

• the sample tests won't run, they have syntax errors

• the description isn't clear on the order of the validation

• the description also doesn't give the correct expected return value