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.

.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 +70,33 @@ lint.isort.force-sort-within-sections = true
 lint.isort.order-by-type = false
 lint.pydocstyle.convention = "google"
 
+[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
+
+    [coverage:run]
+    plugins = covdefaults
+    source =
+        btrfs2s3
+        tests
+"""
+
 [tool.mypy]
 mypy_path = "typeshed"
 strict = true