DOC add Python2.7 warning to recent whatsnew; include 23.3 (#21944)

(cherry picked from commit 4802002)

Author: h-vetinari

doc/source/whatsnew/v0.23.1.txt

@@ -6,6 +6,11 @@ v0.23.1
 This is a minor bug-fix release in the 0.23.x series and includes some small regression fixes
 and bug fixes. We recommend that all users upgrade to this version.
 
+.. warning::
+
+   Starting January 1, 2019, pandas feature releases will support Python 3 only.
+   See :ref:`install.dropping-27` for more.
+
 .. contents:: What's new in v0.23.1
     :local:
     :backlinks: none

doc/source/whatsnew/v0.23.2.txt

@@ -11,6 +11,10 @@ and bug fixes. We recommend that all users upgrade to this version.
    Pandas 0.23.2 is first pandas release that's compatible with
    Python 3.7 (:issue:`20552`)
 
+.. warning::
+
+   Starting January 1, 2019, pandas feature releases will support Python 3 only.
+   See :ref:`install.dropping-27` for more.
 
 .. contents:: What's new in v0.23.2
     :local:

doc/source/whatsnew/v0.23.4.txt

@@ -6,6 +6,10 @@ v0.23.4
 This is a minor bug-fix release in the 0.23.x series and includes some small regression fixes
 and bug fixes. We recommend that all users upgrade to this version.
 
+.. warning::
+
+   Starting January 1, 2019, pandas feature releases will support Python 3 only.
+   See :ref:`install.dropping-27` for more.
 
 .. contents:: What's new in v0.23.4
     :local: