Mute solc which stderr output

[dmuhs]

Issue

When a solc binary is not installed on the system, the which lookup in the installation routine returns 1 and prints an error message to stdout. This message is visible in client applications, even though it is not critical and a solc executable is correctly downloaded and installed.

A sample error message looks like this:

which: no solc in (/home/kthxbai/tools/mythx-cli/venv/bin:<more paths>)

What I did

Route the stderr of which to /dev/null in the lookup logic for detecting already installed solc versions.

How to verify it

Run any script that dynamically installs a specified solc version and verify that no error message is shown.

Checklist

I have not included a changelog entry as the v0.9.0 PR is already open and you might want to include this in the upcoming release by merging this PR into master and rebasing into the PR branch.

• [x] I have confirmed that my PR passes all linting checks
• [ ] I have added an entry to the changelog

[codecov[bot]]

Codecov Report

Merging #81 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #81   +/-   ##
=======================================
  Coverage   76.97%   76.97%           
=======================================
  Files           8        8           
  Lines         582      582           
  Branches      133      133           
=======================================
  Hits          448      448           
  Misses        105      105           
  Partials       29       29           

Impacted Files	Coverage Δ
solcx/install.py	73.20% <100.00%> (ø)

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