Conditionally depend on imgconverter for newer versions of Sphinx
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4b10a580512..1b8032fa1d2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,7 @@ Certbot adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 
 * Update code and dependencies to clean up Resource and Deprecation Warnings.
+* Only depend on imgconverter extension for Sphinx >= 1.6
 
 Despite us having broken lockstep, we are continuing to release new versions of
 all Certbot components during releases for the time being, however, the only
diff --git a/docs/conf.py b/docs/conf.py
index 2e6c5a9b71d..c72d1c1cff1 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -17,6 +17,8 @@
 import re
 import sys
 
+import sphinx
+
 
 here = os.path.abspath(os.path.dirname(__file__))
 
@@ -33,14 +35,13 @@
 # -- General configuration ------------------------------------------------
 
 # If your documentation needs a minimal Sphinx version, state it here.
-needs_sphinx = '1.0'
+needs_sphinx = '1.2'
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 extensions = [
     'sphinx.ext.autodoc',
-    'sphinx.ext.imgconverter',
     'sphinx.ext.intersphinx',
     'sphinx.ext.todo',
     'sphinx.ext.coverage',
@@ -48,6 +49,9 @@
     'repoze.sphinx.autointerface',
 ]
 
+if sphinx.version_info >= (1, 6):
+    extensions.append('sphinx.ext.imgconverter')
+
 autodoc_member_order = 'bysource'
 autodoc_default_flags = ['show-inheritance', 'private-members']
 
diff --git a/setup.py b/setup.py
index f8f5feadc40..90e68680dcb 100644
--- a/setup.py
+++ b/setup.py
@@ -68,9 +68,10 @@ def read_file(filename, encoding='utf8'):
 ]
 
 docs_extras = [
+    # If you have Sphinx<1.5.1, you need docutils<0.13.1
+    # https://github.com/sphinx-doc/sphinx/issues/3212
     'repoze.sphinx.autointerface',
-    # sphinx.ext.imgconverter
-    'Sphinx >=1.6',
+    'Sphinx>=1.2', # Annotation support
     'sphinx_rtd_theme',
 ]
 
diff --git a/tools/dev_constraints.txt b/tools/dev_constraints.txt
index 65182bcded5..778012d31d3 100644
--- a/tools/dev_constraints.txt
+++ b/tools/dev_constraints.txt
@@ -14,7 +14,7 @@ coverage==4.4.2
 decorator==4.1.2
 dns-lexicon==2.7.14
 dnspython==1.15.0
-docutils==0.14
+docutils==0.12
 execnet==1.5.0
 future==0.16.0
 futures==3.1.1

Fixes #6343.

Be sure to edit the `master` section of `CHANGELOG.md` with a line describing this PR before it gets merged.
