Remove pipenv dependency

[SethMMorton]

pipenv was added a development tool to natsort because I got caught up in all the hype. Turns out, for developing libraries pipenv offers almost no benefit over just defining a requirements.txt for your dependencies. So, I am removing pipenv as a dependency.

For people who are using tox, there will be no observable change. For people not using tox but just building natsort (and not running tests) there will be no observable change. This will only affect users who are runningnatsort tests outside of tox. For those users, they will just change from

$ pip install pipenv
$ pipenv install --dev --skip-lock
$ pipenv run python -m pytest

to

$ pip install -r dev-requirements.txt
$ python -m pytest

Much simpler! It also lets the user dictate how they handle virtual environments.

[codecov]

Codecov Report

Merging #86 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #86   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          12       12           
  Lines         623      623           
=======================================
  Hits          620      620           
  Misses          3        3

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