Make pylint respect libraries installed into VIRTUAL_ENV

Jedi respects VIRTUAL_ENV environment variable at finding out
libraries. Therefore, (virtualenv) runtime for pyls/jedi can be
separated from one for the target workspace.

On the other hand, pylint does not respect VIRTUAL_ENV, and might
cause unintentional "import-error" (E0401) for libraries installed in
such virtualenv, even though jedi can recognize them.

In order to make pylint respect libraries installed into VIRTUAL_ENV,
this commit uses Document.sys_path() instead of sys.path of current
pyls process, at spawning pylint. Document.sys_path() should respect
VIRTUAL_ENV, (original) PYTHONPATH, and so on, because it uses
Environment.get_sys_path() of jedi.

This commit chooses changing pyls instead of pylint, because pylint
uses "astroid" library to find out libraries imported in the target
file, and making astroid respect VIRTUAL_ENV seems very difficult (at
least, not so easy).

pyls/plugins/pylint_lint.py

@@ -16,9 +16,19 @@
 
 if sys.version_info.major == 2:
     from StringIO import StringIO
+
+    # subprocess.Popen() on Windows expects that env contains only
+    # "str" keys/values (= "bytes" for Python2.x, "unicode" for
+    # Python3.x), even though pyls treats all path values as "unicode"
+    # regardless of Python version
+    def stringify(u):
+        return u.encode('utf-8')
 else:
     from io import StringIO
 
+    def stringify(u):
+        return u
+
 log = logging.getLogger(__name__)
 
 
@@ -33,7 +43,7 @@ def spawn_pylint(document, flags):
         path = path.replace('\\', '/')
 
     env = dict(os.environ)
-    env["PYTHONPATH"] = os.pathsep.join(sys.path)
+    env["PYTHONPATH"] = stringify(os.pathsep.join(document.sys_path()))
 
     # Detect if we use Python as executable or not, else default to `python`
     executable = sys.executable if "python" in sys.executable else "python"