switch to legacy_tox_ini, remove tox-ini-fmt.

I noticed tox-dev/tox-ini-fmt#42, which makes
it hard to use comments in tox.ini. Auto-formatting tox.ini was the main
motivation to use a separate tox.ini at all, so just keep everything in
pyproject.toml.

Maybe tox will eventually have first-party support for pyproject.toml;
this will make the transition that much simpler.

It appears that tox==3.21.4 (appears on on ubuntu-22.04) can't actually
parse the legacy_tox_ini option, and so does not get to the point of
provisioning a tox environment for the approrpiate version. We now do
proper provisioning ourselves in github ci.

.github/workflows/tests.yaml

@@ -15,9 +15,12 @@ jobs:
     - uses: actions/checkout@v4
     - run: |
         sudo apt update
-        sudo apt install python3-btrfsutil tox
-    - run: |
-        sudo tox
+        sudo apt install python3-btrfsutil python3-virtualenv
+        python3 -m virtualenv /tmp/venv
+        source /tmp/venv/bin/activate
+        python3 -m pip install --upgrade pip
+        python3 -m pip install --upgrade tox
+        sudo -E env PATH=$PATH tox
 
   pre-commit:
     runs-on: ubuntu-latest

.pre-commit-config.yaml

@@ -21,10 +21,6 @@ repos:
   rev: 2.1.3
   hooks:
   - id: pyproject-fmt
-- repo: https://github.com/tox-dev/tox-ini-fmt
-  rev: 1.3.1
-  hooks:
-  - id: tox-ini-fmt
 - repo: https://github.com/google/yamlfmt
   rev: v0.12.1
   hooks:

pyproject.toml

@@ -70,6 +70,36 @@ lint.isort.force-sort-within-sections = true
 lint.isort.order-by-type = false
 lint.pydocstyle.convention = "google"
 
+[tool.coverage.run]
+plugins = [
+  "covdefaults",
+]
+source = [
+  "btrfs2s3",
+  "tests",
+]
+
+[tool.tox]
+legacy_tox_ini = """
+  [tox]
+  requires =
+      tox>=4.2
+  env_list =
+      py
+
+  [testenv]
+  system_site_packages = true
+  deps =
+      covdefaults
+      coverage
+      moto[s3]
+      pytest
+  commands =
+      coverage erase
+      coverage run -m pytest {posargs}
+      coverage report
+"""
+
 [tool.mypy]
 mypy_path = "typeshed"
 strict = true