switch to pip3 if pip is not available in path

[manzanillo]

Use subprocess to handle any error that comes from having pip not defined in path. In those cases, it will use pip3.

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Test results

    6 files  ±0      6 suites  ±0   1m 38s :stopwatch: +4s 262 tests ±0  261 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0    1 :fire: ±0  807 runs  ±0  783 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  24 :fire: ±0 

For more details on these errors, see this check.

Results for commit 7b9f0927. ± Comparison against base commit 03c48e3c.

:recycle: This comment has been updated with latest results.

[qpavsmi]

@cla-bot check

[cla-bot[bot]]

The cla-bot has been summoned, and re-checked this pull request!

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