use environment var 'PIP_UPGRADER_SKIP_PACKAGE_INSTALLATION' to skip…

[umeboshi2]

… package installs

I used the variable name you suggested. The code only checks for the existence of the variable, but not the set value. The messages were changed slightly to indicate "dry run" or "skip install," with "dry run" having priority. Also included a couple of minor pep8 cosmetic changes. Please let me know if you need anything handled differently.

Please don't accept until I run tox. I only have py27 and py35 on system. LOL!, I forgot to unset envvar when running tox and the tests failed! I should create a test for this PR.

[coveralls]

Coverage increased (+0.005%) to 99.697% when pulling 087319f on umeboshi2:skip-pkg-install-env into 3cde8a3 on simion:master.

[umeboshi2]

@simion , I didn't have much time yesterday afternoon to look at things, but when I looked today, it seems that having a different pypi mirror in ~/.pip/pip.conf was causing all of the tests to fail. I'm using a devpi mirror on my laptop. When I removed the config, the tests passed. I looked at the tests to find where the "skip install" option is being tested, so I could add a test for using the env var, but it seems that "skip install" is not being tested. I am not sure what to do about the loss of coverage, but perhaps adding a test for "skip install" would help this.

[simion]

Thank you for the contribution! 🤗

[simion]

I've released 1.4.15 with your fix

[umeboshi2]

Thanks!