ADR: only use Python 3.6+

Document the decision drop support for EOL Python versions, most notable
Python 2.7

Fixes theupdateframework#1125

Signed-off-by: Joshua Lock <[email protected]>

docs/adr/0001-python-version-3-6-plus.md

@@ -0,0 +1,39 @@
+# Default to Python 3.6 or newer for new development
+
+* Status: accepted
+* Date: 2020-10-20
+
+Technical Story: https://github.com/theupdateframework/tuf/issues/1125
+
+## Context and Problem Statement
+
+We do not want to try and support end-of-life versions of the language.
+We want to use modern language features, such as typing.
+We want to ease maintainer burden, by reducing the major language versions supported.
+
+## Decision Drivers
+
+* Python 2.7 is end-of-life
+* Python 3.5 is end-of-life
+* Modern Python allows use of desirable features such as type hints
+* Supporting end-of-life Python versions adds maintenance overhead
+
+## Considered Options
+
+* Support Python 2.7 and 3.5+
+* Support Python 2.7 and 3.6+
+* Support Python 2.7 and 3.6+ (with polyfill modules)
+* Support only Python 3.6+
+
+## Decision Outcome
+
+Chosen option: "Support only Python 3.6+", because we want modern features and lower
+maintainer effort as we work to improve our codebase through the refactor effort.
+
+Using modules to polyfill standard library features from Python 3.6+ feels
+untenable as more libraries are dropping support for EOL Python releases.
+
+### Negative Consequences
+
+* Leaves major adopter and contributor without an actively developed client for some of
+  their customers stuck on older Python versions.

docs/adr/index.md

@@ -4,7 +4,8 @@ This log lists the architectural decisions for tuf.
 
 <!-- adrlog -- Regenerate the content by using "adr-log -i". You can install it via "npm install -g adr-log" -->
 
-- [ADR-0000](0000-use-markdown-architectural-decision-records.md) - Use Markdown Architectural Decision Records
+- [ADR-0000](../adr0000-use-markdown-architectural-decision-records.md) - Use Markdown Architectural Decision Records
+- [ADR-0001](../adr0001-python-version-3-6-plus.md) - Default to Python 3.6 or newer for new development
 
 <!-- adrlogstop -->