Always return float value for mean_backoff_time

[tlinhart]

Always return mean_backoff_time as a float number. Returning both int and float types can cause various problems, one example being field type conflict when storing stats values in databases like InfluxDB which have fixed schema. If int value comes first, there can't be written float value anytime after as the field already exists as an integer.

[codecov-io]

Codecov Report

Merging #25 into master will not change coverage. The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #25   +/-   ##
=======================================
  Coverage   51.68%   51.68%           
=======================================
  Files           5        5           
  Lines         238      238           
  Branches       39       39           
=======================================
  Hits          123      123           
  Misses        109      109           
  Partials        6        6

Impacted Files	Coverage Δ
rotating_proxies/expire.py	78.88% <0%> (ø)	:arrow_up:

[kmike]

Makes sense, thanks @tlinhart!