Upgrade to pip 19#13

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cjolowicz wants to merge 9 commits intomasterfrom

ci-upgrade-to-pip-19

Commits on Nov 24, 2019

Switch Heroku account used by Travis CI
Claudio Jolowicz
committed
Use a separate CI job per stack
Claudio Jolowicz
committed
Allow running specific tests by passing TESTS
Claudio Jolowicz
committed
Skip undefined functions when processing TESTS
Claudio Jolowicz
committed
Do not run all tests when no tests match TESTS
Claudio Jolowicz
committed
Upgrade to pip 19.2.3
Claudio Jolowicz
committed
Upgrade to pip 19.1.1 for Python 3.4 projects
Claudio Jolowicz
committed
Adapt pip-pop to changes in pip >= 10
Claudio Jolowicz
committed
Pin to pip 9.0.2 for pipenv users only
Claudio Jolowicz
committed