Ensure environs are strings on Python2 + Windows

Patch based on theskumar/python-dotenv#101 by @greyli.

pipenv/vendor/dotenv/main.py

@@ -94,6 +94,14 @@ def set_as_environment_variables(self, override=False):
         for k, v in self.dict().items():
             if k in os.environ and not override:
                 continue
+            # With Python2 on Windows, force environment variables to str to
+            # avoid "TypeError: environment can only contain strings" in
+            # Python's subprocess module.
+            if sys.version_info.major < 3 and sys.platform == 'win32':
+                if not isinstance(k, str):
+                    k = k.encode(sys.getdefaultencoding())
+                if not isinstance(v, str):
+                    v = v.encode(sys.getdefaultencoding())
             os.environ[k] = v
 
         return True

tasks/vendoring/patches/vendor/dotenv-windows-unicode.patch

@@ -0,0 +1,19 @@
+diff --git a/pipenv/vendor/dotenv/main.py b/pipenv/vendor/dotenv/main.py
+index 3d1bd72f..4e1b4b46 100644
+--- a/pipenv/vendor/dotenv/main.py
++++ b/pipenv/vendor/dotenv/main.py
+@@ -94,6 +94,14 @@ class DotEnv():
+         for k, v in self.dict().items():
+             if k in os.environ and not override:
+                 continue
++            # With Python2 on Windows, force environment variables to str to
++            # avoid "TypeError: environment can only contain strings" in
++            # Python's subprocess module.
++            if sys.version_info.major < 3 and sys.platform == 'win32':
++                if not isinstance(k, str):
++                    k = k.encode(sys.getdefaultencoding())
++                if not isinstance(v, str):
++                    v = v.encode(sys.getdefaultencoding())
+             os.environ[k] = v
+
+         return True