add warning about Python2.7 support drop (scikit-learn#11545)

* add warning about Python2.7 support drop

* Add python27 drop to readme, rephrase whatsnew

* add warning to install.rst

* add  0.20 to news, add remark that 0.21 will drop 2.7

* Mention 3.4

* fix

* also mention 3.4

* DOC: note in bold

Author: amueller

README.rst

@@ -53,6 +53,9 @@ scikit-learn requires:
 - NumPy (>= 1.8.2)
 - SciPy (>= 0.13.3)
 
+**Scikit-learn 0.20 is the last version to support Python2.7.**
+Scikit-learn 0.21 and later will require Python 3.5 or newer.
+
 For running the examples Matplotlib >= 1.3.1 is required. A few examples
 require scikit-image >= 0.9.3 and a few examples require pandas >= 0.13.1.
 

doc/index.rst

@@ -207,6 +207,10 @@
                     <li><em>On-going development:</em>
                     <a href="/dev/whats_new.html"><em>What's new</em> (Changelog)</a>
                     </li>
+                    <li><strong>Scikit-learn 0.21 will drop support for Python 2.7 and Python 3.4.</strong>
+                    </li>
+                    <li><em>July 2018.</em> scikit-learn 0.20 is available for download (<a href="whats_new.html#version-0-20">Changelog</a>).
+                    </li>
                     <li><em>July 2018.</em> scikit-learn 0.19.2 is available for download (<a href="whats_new.html#version-0-19">Changelog</a>).
                     </li>
                     <li><em>October 2017.</em> scikit-learn 0.19.1 is available for download (<a href="whats_new.html#version-0-19">Changelog</a>).
@@ -215,12 +219,6 @@
                     </li>
                     <li><em>June 2017.</em> scikit-learn 0.18.2 is available for download (<a href="whats_new/v0.18.html#version-0-18-2">Changelog</a>).
                     </li>
-                    <li><em>September 2016.</em> scikit-learn 0.18.0 is available for download (<a href="whats_new/v0.18.html#version-0-18">Changelog</a>).
-                    </li>
-                    <li><em>November 2015.</em> scikit-learn 0.17.0 is available for download (<a href="whats_new/v0.17.html">Changelog</a>).
-                    </li>
-                    <li><em>March 2015.</em> scikit-learn 0.16.0 is available for download (<a href="whats_new/v0.16.html">Changelog</a>).
-                    </li>
                     </ul>
                 </div>
 

doc/install.rst

@@ -21,6 +21,12 @@ Scikit-learn requires:
 - NumPy (>= 1.8.2),
 - SciPy (>= 0.13.3).
 
+
+.. warning::
+
+    Scikit-learn 0.20 is the last version to support Python 2.7 and Python 3.4.
+    Scikit-learn 0.21 will require Python 3.5 or newer.
+
 If you already have a working installation of numpy and scipy,
 the easiest way to install scikit-learn is using ``pip`` ::
 

doc/whats_new/v0.20.rst

@@ -11,6 +11,11 @@ This release packs in a mountain of bug fixes, features and enhancements for
 the Scikit-learn library, and improvements to the documentation and examples.
 Thanks to our many contributors!
 
+.. warning::
+
+    Version 0.20 is the last version of scikit-learn to support Python 2.7 and Python 3.4.
+    Scikit-learn 0.21 will require Python 3.5 or higher.
+
 Highlights
 ----------