Fix indentation levels

more helpful messages about supported versions

With inspiration from @KevinBrolly, this patch uses the stack image
SQLite3 package but also still providing the dev headers and binary that
users may still be using today. The benefit is that we won't need to
rebuild all the python binaries for this to take affect. We can just
stop shipping SQLite3 from future binaries. In addition, we don't need
to worry about what version and when to update SQLite3 and maintaining
the packages ourselves.

This also includes updates to Python 2.7.15 and Python 3.6.6 so they can
rebuilt with the stack image dev headers instead of building our own
vendored SQLite3.

Python 3.7.0 is supported but not preferred given how new it is. As a
result, we don't want it to be the default, but we also don't want users
to be confused when upgrading to it.

Closes gh-728

New warnings were found in our periodic (CRON) CI jobs.

Fixes https://travis-ci.org/heroku/heroku-buildpack-python/jobs/383404864

* freeze pip version, upon installation

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* use PIP_UPDATE environment variable

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* disable shell checking (for now)

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* --disable-pip-version-check

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* Since pipenv is being pinned now, output pinned version in steps

Signed-off-by: Yury V. Zaytsev <yury.zaytsev@moneymeets.com>

* Update README.md example output to match new reality

Signed-off-by: Yury V. Zaytsev <yury.zaytsev@moneymeets.com>

* Update pipenv

* default to user python path for collectstatic runs

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* bugfixes

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* fix

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* metrics

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* using pipenv anyway

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* instrument bad failures

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* if not

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* Update pipenv-python-version

* use latest versions in pipenv script

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* Update pipenv-python-version

* 3.6.4

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* Fix PYTHONPATH is not set when running collectstatic

* update changelog

Signed-off-by: Kenneth Reitz <me@kennethreitz.org>

* Update CHANGELOG.md

This is useful if you'd like to customize python binaries without forking the entire buildpack.

Similar functionality was implemented in https://github.com/heroku/heroku-buildpack-ruby/pull/238

* Add support for PIP_EXTRA_INDEX_URL

* Add support for PIP_EXTRA_INDEX_URL for Pipenv