Add support for .python-version files

src/hooks/useVirtualEnv.ts

@@ -142,6 +142,23 @@ export default async function(cwd: Path = Path.cwd()): Promise<VirtualEnv> {
         throw new Error('couldn’t parse: .node-version')
       }
     }
+    if (_if(".python-version")) {
+      const s = (await f!.read()).trim()
+      const lines = s.split("\n")
+      for (let l of lines) {
+        l = l.trim()
+        if (!l) continue  // skip empty lines
+        if (l.startsWith('#')) continue  // skip commented lines
+        // TODO: How to handle 'system'?
+        // TODO: How to handle non-bare versions like pypy3.9-7.3.11, stackless-3.7.5, etc. in pyenv install --list?
+        l = `python.org@${l}`
+        try {
+          pkgs.push(pkg.parse(l))
+        } catch {
+          throw new Error('couldn’t parse: .python-version')
+        }
+      }
+    }
     if (_if("package.json")) {
       const json = JSON.parse(await f!.read())
       if (isPlainObject(json?.tea)) {

tests/fixtures/.python-version

@@ -0,0 +1 @@
+3.10

tests/functional/devenv.test.ts

@@ -124,6 +124,26 @@ Deno.test("should provide packages in dev env", { sanitizeResources: false, sani
   }
 })
 
+Deno.test("should provide python in dev env", { sanitizeResources: false, sanitizeOps: false }, async test => {
+  const SHELL = "/bin/zsh"
+
+  const tests = [
+    { file: ".python-version", pkg: "python.org~3.10" }
+  ]
+
+  for (const { file, pkg } of tests) {
+    await test.step(file, async () => {
+      const { run, teaDir } = await createTestHarness()
+
+      fixturesDir.join(file).cp({ into: teaDir })
+      const { stdout } = await run(["+tea.xyz/magic", "-Esk", "--chaste", "env"], { env: { SHELL } })
+
+      const output = getTeaPackages(SHELL, stdout)
+      assert(output.includes(pkg), "should include python dep")
+    })
+  }
+})
+
 Deno.test("tolerant .node-version parsing", { sanitizeResources: false, sanitizeOps: false }, async test => {
   const SHELL = "/bin/zsh"