Update to include latest Pandas version, bring tests to 100% coverage, remove support for Python 3.8 and include for 3.11, others

[shaunporwal]

• cast harm_column as float
• use .iloc[0] specifically extract first element from pandas series
• use .iloc[0] to extract first elements in pandas series in prevalence calculation
• update Link in readme to decisioncurveanalysis.org rather than Dan's site
• create first NEWS.md file using towncrier
• add ignore::DeprecationWarning for packages generating deprecation warnings due to using pkg_resources
• remove explicit pkg_resources dependency in favor of importlib_resources
• remove support for python 3.8 and allow 3.9-3.11
• brings tests to 100% coverage
• allow for >=1.53 to <3 versions of pandas
• start implementing lowest smoothing in unit tests