add default_python replacement to sphinx

and get docs version from the package so it doesn't get out of date

Author: minrk

docs/source/conf.py

@@ -80,9 +80,19 @@ def setup(app):
 # built documents.
 #
 # The short X.Y version.
-version = '0.1'
+import repo2docker
+version = repo2docker.__version__
 # The full version, including alpha/beta/rc tags.
-release = '0.1'
+release = version
+
+from repo2docker.buildpacks.conda import CondaBuildPack
+
+default_python = CondaBuildPack.major_pythons['3']
+
+rst_prolog = """
+.. |default_python| replace:: **Python {default_python}**
+.. |default_python_version| replace:: {default_python}
+""".format(default_python=default_python)
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.