Replaced use of pip internal function.

[tannart]

As discussed in this thread: pypa/pip#5243
use of the functions inside pip._internal is strongly discouraged. As
such I have replaced this functions usage with the pip maintainers
recommended solution.

[codecov]

Codecov Report

Merging #127 into master will decrease coverage by 0.44%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
- Coverage   68.09%   67.64%   -0.45%     
==========================================
  Files           7        7              
  Lines         304      306       +2     
==========================================
  Hits          207      207              
- Misses         97       99       +2

Impacted Files	Coverage Δ
safety/cli.py	47.05% <0%> (-1.93%)	⬇️

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[Jwomers]

Thanks, closing this, going forward with #151 which solves the same problem.