Upgrade minimum Python version for FloPy 3.3.5

[mwtoews]

The current minimum is Python 3.5 (PEP 478), which reached its end-of-life in 2020, and has been retired. I'd suggest this be bumped up to Python 3.7 (PEP 537) (note that Python 3.6 - PEP 494 - will reach its end-of-life at the end of 2021). There are many enhancements available in Python 3.6 and Python 3.7 that would benefit the code base, including:

• f-strings should be used where possible, particularly for new code. They are easier to see what is being formatted, and have less overhead than .format().
• dict objects are much more efficient since 3.6, and guarantee insertion order in 3.7, meaning things like OrderedDict objects are no longer necessary.

Seeking feedback if a different minimum version of Python should instead be targeted, otherwise I can prepare a pull request to close this issue.