Due to how the version checks work - via sorting, not actual comparison - this previously resulted in _always_ installing sqlite3, even though it was already bundled for lower versions of python. The second version check also encompasses 3.7.0+, so there is no need to respecify the check.

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

Also update our documentation and CHANGELOG for this version of the
buildpack.

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

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

Caught these with the re-added shellcheck linting.

* 3.6.5

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

* fix tests

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

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

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

* Add Python 2.7.15 to the list of runtimes

Closes #691

* Update the default Python 2 to 2.7.15 everywhere

* 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>

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>