fix: support pip 18

the `get_installed_distributions` method that was moved under `._internal`,
was moved again in pip 18 to `._internal.utils.misc`.
This PR tries the new location if the 2 existing options don't work.

.travis.yml

@@ -10,10 +10,10 @@ node_js:
 env:
   - PIP_VER=9.0.3   PYTHON_VER=2.7
   - PIP_VER=9.0.3   PYTHON_VER=3.6
-#  - PIP_VER=9.0.3   PYTHON_VER=3.7  # missing in Travis's old version of pyenv
   - PIP_VER=10.0.0  PYTHON_VER=2.7
   - PIP_VER=10.0.0  PYTHON_VER=3.6
-#  - PIP_VER=10.0.0  PYTHON_VER=3.7  # missing in Travis's old version of pyenv
+  - PIP_VER=18.1.0  PYTHON_VER=2.7
+  - PIP_VER=18.1.0  PYTHON_VER=3.6
 cache:
   directories:
     - node_modules

plug/pip_resolve.py

@@ -9,12 +9,17 @@
 import pipfile
 
 # pip >= 10.0.0 moved all APIs to the _internal package reflecting the fact
-# that pip does not currently have any public APIs. This is a temporary fix.
-# TODO: We need a workaround to using the get_installed_distributions method.
+#   that pip does not currently have any public APIs.
+# pip >= 18.0.0 moved the internal API we use deeper to _internal.utils.misc
+# TODO: This is a temporary fix that might not hold again for upcoming releases,
+#   we need a better approach for this.
 try:
     from pip import get_installed_distributions
 except ImportError:
-    from pip._internal import get_installed_distributions
+    try :
+        from pip._internal import get_installed_distributions
+    except ImportError:
+        from pip._internal.utils.misc import get_installed_distributions
 
 
 def create_tree_of_packages_dependencies(dist_tree, packages_names, req_file_path, allow_missing=False):