switch from cdecimal to decimal for python >=3.3

[dlaehnemann]

When setting up scVILP, I ran into dependency solver issues with conda, because I was trying to get scVILP (and gurobi) to work with python 3.6 or 3.7. Apparently, cdecimal doesn't exist any more for those versions, as it has been integrated into python itself as the standard module decimal. See e.g. here:

• https://pypi.org/project/cdecimal/

  Please note: cdecimal has been integrated into CPython 3.3, where it supersedes the pure Python version: import decimal will automatically import the C version. Performance has been improved further, so the cdecimal version shipped with CPython 3.3 is significantly faster for numerical workload than cdecimal-2.3. If you need maximum decimal computing performance, you should solely use that Python version.

• http://www.bytereef.org/mpdecimal/index.html

  cdecimal has been included in Python-3.3 and later, just “import decimal”.