When using pipenv, install packages (such as linters, test frameworks) in dev-packages

[DonJayamanne]

Fixes #1110

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

Merging #1134 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #1134     +/-   ##
=========================================
- Coverage   71.81%   71.72%   -0.1%     
=========================================
  Files         260      260             
  Lines       11898    11898             
  Branches     2112     2112             
=========================================
- Hits         8545     8534     -11     
- Misses       3226     3232      +6     
- Partials      127      132      +5

Impacted Files	Coverage Δ
src/client/common/installer/pipEnvInstaller.ts	72.22% <100%> (ø)	⬆️
src/client/debugger/PythonProcess.ts	43.33% <0%> (-2.09%)	⬇️
...rc/client/debugger/PythonProcessCallbackHandler.ts	51.64% <0%> (-1.98%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 287fd8b...4a0f8f3. Read the comment docs.