Fix dependency resolution for Python-restricted dependencies

poetry/puzzle/provider.py

@@ -331,7 +331,7 @@ class Provider:
                 # In this particular case, we notify the resolver that it needs
                 # to branch the dependency tree. What this means is if we have
                 # root Python ~2.7 || ^3.6 and dependency Python >=3.6
-                # we have to resolve for ^3.6 (>=3.6, <4.0)
+                # we have to resolve for ^3.6 (>=3.6, <4.0) and for ~2.7 || <3.6
                 if (
                     not package.dependency.python_constraint.is_any()
                     and not package.python_constraint.intersect(
@@ -343,12 +343,12 @@ class Provider:
                             package, package.dependency.python_constraint
                         )
                     )
+                    intersection = self._package.python_constraint.intersect(
+                        package.dependency.python_constraint
+                    )
                     raise CompatibilityError(
-                        str(
-                            package.python_constraint.intersect(
-                                package.dependency.python_constraint
-                            )
-                        )
+                        str(intersection),
+                        str(self._package.python_constraint.difference(intersection)),
                     )
 
                 return [

pyproject.toml

@@ -22,7 +22,7 @@ classifiers = [
 
 # Requirements
 [tool.poetry.dependencies]
-python = "~2.7.9 || ^3.4"
+python = "~2.7 || ^3.4"
 cleo = "^0.6.7"
 requests = "^2.18"
 cachy = "^0.2"
@@ -47,7 +47,7 @@ virtualenv = { version = "^16.0", python = "~2.7" }
 [tool.poetry.dev-dependencies]
 pytest = "^3.4"
 pytest-cov = "^2.5"
-mkdocs = "^1.0"
+mkdocs = { version = "^1.0", python = "~2.7.9 || ^3.4" }
 pymdown-extensions = "^4.9"
 pygments = "^2.2"
 pytest-mock = "^1.9"