Add support for very old versions of setuptools

[SethMMorton]

The syntax "fastnumbers >= 2.0.0; python_version > 2.6" is "new" in the
sense that there are still OSs (like CentOS) that use still have
setuptools versions installed that do not support it.

The solution is to selectively add the dependency using logic rather
than declaratively.

This will solve issue #64.

[codecov]

Codecov Report

Merging #84 into master will decrease coverage by 8.29%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #84     +/-   ##
=========================================
- Coverage   99.52%   91.22%   -8.3%     
=========================================
  Files          12       12             
  Lines         627      627             
=========================================
- Hits          624      572     -52     
- Misses          3       55     +52

Impacted Files	Coverage Δ
natsort/compat/locale.py	48.21% <0%> (-51.79%)	⬇️
natsort/compat/py23.py	79.54% <0%> (-13.64%)	⬇️
natsort/natsort.py	87% <0%> (-13%)	⬇️
natsort/compat/fastnumbers.py	87.5% <0%> (-12.5%)	⬇️
natsort/__init__.py	91.66% <0%> (-8.34%)	⬇️
natsort/utils.py	99.2% <0%> (-0.8%)	⬇️

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