Require enum34 only under Python 2.7

It seems like Ubuntu trusty's version of pip
does in fact support environment markers.
At least it works for mitmproxy:
https://github.com/mitmproxy/mitmproxy/blob/deb7844/setup.py#L93-L98

I assume it will also work in Debian 8 "jessie"
which has even newer python-{pip,setuptools} than trusty.

MANIFEST.in

@@ -1,4 +1,3 @@
-include requirements.in
 include setup.cfg
 
 include README.rst

setup.py

@@ -13,10 +13,6 @@
 with io.open('README.rst') as f:
     long_description = f.read()
 
-with io.open('requirements.in') as f:
-    install_requires = [line for line in f
-                        if line and not line.startswith('#')]
-
 
 setup(
     name='HTTPolice',
@@ -27,7 +23,23 @@
     author='Vasiliy Faronov',
     author_email='vfaronov@gmail.com',
     license='MIT',
-    install_requires=install_requires,
+
+    # XXX: when updating these fields,
+    # make sure you don't break ``tools/minimum_requires.sh``.
+    install_requires=[
+        'singledispatch >= 3.4.0.3',
+        'six >= 1.10.0',
+        'lxml >= 3.6.0',
+        'bitstring >= 3.1.4',
+        'dominate >= 2.2.0',
+        'defusedxml >= 0.4.1',
+    ],
+    extras_require={
+        ':python_version == "2.7"': [
+            'enum34 >= 1.1.6',
+        ],
+    },
+
     packages=[
         'httpolice',
         'httpolice.inputs',

tools/minimum_requires.sh

@@ -1,11 +1,12 @@
 #!/bin/sh
 
-# Install the lowest versions of requirements permitted by HTTPolice.
-# This checks that these lower bounds are up-to-date.
+# Install the lowest versions of requirements permitted by HTTPolice,
+# so we can ensure that these lower bounds are up-to-date.
 
 set -e
 
 test -e setup.py || { echo 'must run from repo root' >&2; exit 1; }
 
-sed -e 's/>=/==/g' <requirements.in >minimum_requirements.txt
+grep -Eo '[A-Za-z0-9_.]+ >= [A-Za-z0-9_.]+' setup.py | \
+    sed -e 's/>=/==/g' >minimum_requirements.txt
 pip install -r minimum_requirements.txt