Environment sanity checks

[sbidoul]

closes #37

• abort if pip is absent
• abort if pkg_resources is absent
• warn if old pip
• warn if wheel is absent

[codecov]

Codecov Report

Merging #40 into master will increase coverage by 0.10%.
The diff coverage is 95.91%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   93.93%   94.03%   +0.10%     
==========================================
  Files          14       15       +1     
  Lines         758      805      +47     
  Branches      140      143       +3     
==========================================
+ Hits          712      757      +45     
- Misses         29       30       +1     
- Partials       17       18       +1     

Impacted Files	Coverage Δ
src/pip_deepfreeze/__main__.py	90.47% <60.00%> (-4.40%)	⬇️
src/pip_deepfreeze/sanity.py	100.00% <100.00%> (ø)

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