move tox config to tox.ini (out of pyproject.toml)

We would prefer to configure tox using pyproject.toml, but that's
unfortunately not properly supported yet (there's only a hacky way with
a giant string that we were using before). The problem this causes for us
is that other tools (like Renovate bot) don't support the hacky method,
so we're losing out on updates to our tox config.

According to the issue below (and the tox docs), pyproject will be
properly supported by tox v4, so we should check back then to see if we
can get rid of tox.ini.

see: tox-dev/tox#999
see: tox-dev/tox#814

pyproject.toml

@@ -160,21 +160,3 @@ flake8-black = ["-BLK901"]
 [build-system]
 requires = ["poetry-core>=1.0.0"]
 build-backend = "poetry.core.masonry.api"
-
-[tool.tox]
-# see: https://tox.wiki/en/latest/example/basic.html
-legacy_tox_ini = """
-[tox]
-envlist = py
-isolated_build = True
-
-[testenv]
-deps =
-  pytest >= 6.2
-  pytest-bdd >=4.0.1,<6.0
-  pytest-xdist >=2.5.0
-  toml >=0.10
-
-commands = pytest {posargs}
-passenv = HOME
-"""

tox.ini

@@ -0,0 +1,19 @@
+# We would prefer to use pyproject.toml to configure
+# tox, but it's not properly supported, yet. Check
+# back on this once tox v4 is out.
+# see: https://github.com/tox-dev/tox/issues/999
+# see: https://github.com/tox-dev/tox/issues/814
+# see: https://tox.wiki/en/latest/example/basic.html
+[tox]
+envlist = py
+isolated_build = True
+
+[testenv]
+deps =
+  pytest >= 6.2
+  pytest-bdd >=4.0.1,<6.0
+  pytest-xdist >=2.5.0
+  toml >=0.10
+
+commands = pytest {posargs}
+passenv = HOME