Add `python_requires` stanza to setup.py

This is part of a fix for https://github.com/sybrenstuvel/python-rsa/issues/152

3 files changed, 20 insertions, 0 deletions

diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index f8ed650..3a12e64 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -1,6 +1,16 @@
 Python-RSA changelog
 ========================================
 
+Version 4.3 - released 2020-06-12
+----------------------------------------
+
+Version 4.3 is a re-tagged release of version 4.0. It is the last to support
+Python 2.7. This is now made explicit in the `python_requires` argument in
+`setup.py`.
+
+There are no functional differences.
+
+
 Version 4.0 - released 2018-09-16
 ----------------------------------------
 
diff --git a/README.md b/README.md
index c5e0f2d..b7df4fb 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,15 @@ or download it from the [Python Package Index](https://pypi.org/project/rsa/).
 The source code is maintained at [GitHub](https://github.com/sybrenstuvel/python-rsa/) and is
 licensed under the [Apache License, version 2.0](https://www.apache.org/licenses/LICENSE-2.0)
 
+Changes in 4.1-4.4
+------------------
+
+Version 4.1 dropped support for Python 2.7, and soon after that version 4.2 was released. Neither of the two made it explicit in `setup.cfg` that Python 3.5 or newer is required. This caused issues on Python 2.7, as Pip happily upgraded to the new version.
+
+Version 4.3 is a re-tagged release of version 4.0. It is the last to support Python 2.7.
+
+Version 4.4 will be a re-tagged release of version 4.2, and explicitly require Python 3.5 or newer.
+
 
 Major changes in 4.0
 --------------------
diff --git a/setup.py b/setup.py
index 18f339e..71bf99a 100755
--- a/setup.py
+++ b/setup.py
@@ -51,6 +51,7 @@ if __name__ == '__main__':
               'Programming Language :: Python :: Implementation :: PyPy',
               'Topic :: Security :: Cryptography',
           ],
+          python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, <4',
           install_requires=[
               'pyasn1 >= 0.1.3',
           ],