Revert setup.py portion of "version-in-one-place"

Defining the version string in a single place is a nice goal to have,
but since we are currently straddling the fence as it relates to
setuptools and pyproject.toml, mingling them together results in some
unexpected behavior and is thus not advisable. We can revisit this if
and when we make a full switch away from setuptools.

setup.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python
-import re
 import sys
 from io import open
 from os import walk
@@ -8,21 +7,7 @@ from os.path import join, relpath
 from setuptools import setup
 
 
-def get_version():
-    VERSION_REGEX = re.compile(
-        r"^version\s*=\s*\"(?P<version>.*)\"$"
-    )
-    with open("pyproject.toml") as f:
-        for line in f:
-            match = VERSION_REGEX.match(line)
-
-            if match:
-                return match.group("version")
-
-    return None
-
-
-version = get_version()
+version = "4.0.1"
 
 requires = ['feedgenerator >= 1.9', 'jinja2 >= 2.7', 'pygments', 'docutils',
             'pytz >= 0a', 'blinker', 'unidecode', 'six >= 1.4',