Drop support for developing in a local venv in favour of devcontainers (#249)

We have unofficially been opinionated about devcontainers for a few
months now, and have been directing support requests from people using
venvs to start by making one. Since supporting devcontainers is now a
matter of policy in all-but-name, it should be documented and referred
to in future.

Author: coretl

docs/explanations/decisions/0020-support-devcontainers.md

@@ -0,0 +1,21 @@
+# 20. Support devcontainers as the opinionated way to contribute
+
+Date: 2025-04-07
+
+## Status
+
+Accepted
+
+## Context
+
+The Python copier template is growing more opinionated over time. It includes a `.devcontainer` and `.vscode` directory committed to version control, providing out-of-box configuration for contributors to set up development environments. In general we have preferred contributors to use the devcontainer rather than a local [venv](https://docs.python.org/3/library/venv.html), as it means development takes place in a consistent environment and it is much easier to support contributors and diagnose their problems by locally reproducing their environments. We can also vend a consistent, tested set of recommended vscode plugins.
+
+## Decision
+
+[Devcontainers](https://code.visualstudio.com/docs/devcontainers/containers) are the opinionated way to contribute to projects based on this template. The template will be developed, maintained and tested on the assumption that a contributor is using a devcontainer.
+
+## Consequences
+
+The Python copier template development environment is no longer guaranteed to work outside of a devcontainer. Support is not guaranteed for contributors not using a devcontainer, in the first instance requests for such support should be answered with gentle encouragement to adopt them.
+
+This does not mean non-devcontainer environments are forbidden or designed out, but it does mean the individual contributor is responsible for maintaining such environments, making them work and dealing with any breaking changes themselves. The devcontainer environment is provided for contributors who want something that works out-of-box and is supported, so they can get started quickly. 

docs/how-to/dev-install.md

@@ -8,20 +8,7 @@ First clone the repository locally using [Git](https://git-scm.com/downloads). T
 
 ## Install dependencies
 
-You can choose to either develop on the host machine using a `venv` (which requires python 3.10 or later) or to run in a container under [VSCode](https://code.visualstudio.com/)
-
-<!-- https://sphinx-design.readthedocs.io/en/latest/tabs.html# -->
-::::{tab-set}
-
-:::{tab-item} Local virtualenv
-```
-python3 -m venv venv
-source venv/bin/activate
-pip install -e '.[dev]'
-```
-:::
-
-:::{tab-item} VSCode devcontainer
+It is recommended that developers use a [vscode devcontainer](https://code.visualstudio.com/docs/devcontainers/containers). This repository contains configuration to set up a containerized development environment that suits its own needs.
 
 Ensure you have the [vscode devcontainer extension](https://marketplace.visualstudio.com/items?itemName=ms-vscode-remote.remote-containers) installed.
 
@@ -32,10 +19,8 @@ code .
 ```
 
 Click on 'Reopen in Container' when prompted on startup or, if vscode is already running, open the command menu with CTRL+SHIFT+P, search for and run 'Reopen in Container'.
-Open a new terminal
-:::
 
-::::
+Open a new terminal
 
 ## Build and test