Add support for requirements.in (from pip-tools)

[lvillani]

pip-tools provides a tool called pip-compile that takes a
requirements.in file with the same format used by requirements.txt to produce a "lock file"
(the usual requirements.txt file).

This commits enables the pip-requirements syntax for the aforementioned files.

[brettcannon]

Thanks for hte PR, @lvillani ! We are in code freeze for our next release, but we will look at this after we open the repo back up to feature work.

[codecov]

Codecov Report

Merging #961 into master will increase coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #961      +/-   ##
==========================================
+ Coverage   71.67%   71.89%   +0.21%     
==========================================
  Files         260      260              
  Lines       11877    11877              
  Branches     2104     2104              
==========================================
+ Hits         8513     8539      +26     
+ Misses       3239     3211      -28     
- Partials      125      127       +2

Impacted Files	Coverage Δ
...rc/client/debugger/PythonProcessCallbackHandler.ts	51.64% <0%> (-1.32%)	⬇️
src/client/linters/baseLinter.ts	91.66% <0%> (+4.16%)	⬆️
src/client/common/installer/productInstaller.ts	64.55% <0%> (+4.23%)	⬆️
src/client/common/application/applicationShell.ts	30.76% <0%> (+7.69%)	⬆️
src/client/common/logger.ts	73.33% <0%> (+20%)	⬆️
src/client/linters/errorHandlers/errorHandler.ts	100% <0%> (+22.22%)	⬆️
src/client/linters/errorHandlers/notInstalled.ts	94.44% <0%> (+61.11%)	⬆️

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[brettcannon]

FYI @lvillani the PR looks good, but I need to be in a position to add a news entry before I can merge (unless you want to add a news entry yourself 😁 ).

[lvillani]

Hey @brettcannon, sure I can add the news entry myself. I'm unfamiliar with the process, do I have to create a file called 961.md in the 1 Enhancements directory?

[brettcannon]

@lvillani https://github.com/Microsoft/vscode-python/blob/master/news/README.md should explain what to do (and if it doesn't please let me know as you're the first external contributor to do this 😁 ).

[msftclas]

All CLA requirements met.

[lvillani]

@brettcannon I have added this file and rebased the branch against master. I can also squash all commits together if you want.

[brettcannon]

Thanks, @lvillani !