Require Python >= 3.5

This is a big change.  It will make it possible to simplify the code,
add more features, improve the robustness and lower the barrier to new
contributions.

As per [Python's packaging documentation][doc], the `python_requires`
keyword argument needs `setuptools >= 24.2.0` (released in 2016) and
will only have en effect for `pip >= 9.0.0` (released in 2016 as well).

[doc]: https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, pypy2, pypy3]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9, pypy3]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this
 project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Changed
+
+- Require Python 3.5 or a later version.  Python 2 and 3.4 are no longer supported. (#? by
+  [@bbc2]).
+
 ## [0.18.0] - 2021-06-20
 
 ### Changed

setup.py

@@ -33,6 +33,7 @@ def read_files(files):
     package_data={
         'dotenv': ['py.typed'],
     },
+    python_requires=">=3.5",
     install_requires=[
         "typing; python_version<'3.5'",
     ],