* Fix warnings about escape sequences

* Fix typo in CONTRIBUTING

* Enable Python3.7 in travis

* Document trove classifiers

* Fix incorrect key name

* Resolve markdownlint blank line issues

* Fix typo

This will incorporate the fix for https://github.com/sdispater/tomlkit/issues/10 -- a significant performance bug on Python 2.

This reverts commit 816c5103.

Co-authored-by: Sébastien Eustace <555648+sdispater@users.noreply.github.com>
Co-authored-by: Nikita Grishko <1087619+Gr1N@users.noreply.github.com>

Fixes #349

Fixes #350

* Resolve specified extras of poetry directory dependencies (#282)

* Copy poetry package extras to directory dependency

* Exclude unspecified extras during version resolution

* Add tests in test_installer

* Pass build on Python 3.5

* Remove Python version in test & fixture

As per
https://warehouse.pypa.io/api-reference/legacy/#get--simple--project--,
the project URL should have a trailing slash.

This was discovered as an issue in an edge case situation,
but should be the standard call regardless.

Edge case explanation:
In a private pypi instance built on artifactory,
set up to serve on https (443) and ignore http (80) requests,
artifactory sends a 301 redirect when called without the slash,
but provides the redirect url as http (due to a bug in artifactory).
This results in an almost-valid call (but missing the slash)
failing by being redirected to a nonsecure call that isn't answered.

For whatever it is worth, there is a Bug filed with Artifactory
for the bad redirect (https://www.jfrog.com/jira/browse/RTFACT-14235),
but it is over a year old with no movement.

* fixed unresolved reference

* fixed code formatting