Fix installing entry points

Not sure why or when but the string syntax for entry points does not
seem to work correctly anymore (no scripts are installed).  Use
the explicit list-in-dict syntax instead.

setup.py

@@ -36,10 +36,11 @@ def read_files(files):
     extras_require={
         'cli': ['click>=5.0', ],
     },
-    entry_points='''
-        [console_scripts]
-        dotenv=dotenv.cli:cli
-    ''',
+    entry_points={
+        "console_scripts": [
+            "dotenv=dotenv.cli:cli",
+        ],
+    },
     license='BSD-3-Clause',
     classifiers=[
         'Development Status :: 5 - Production/Stable',