Update tox requirement from <4.19,>=4.16.0 to >=4.16.0,<4.21

Updates the requirements on [tox](https://github.com/tox-dev/tox) to permit the latest version.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/main/docs/changelog.rst)
- [Commits](tox-dev/tox@4.16.0...4.20.0)

---
updated-dependencies:
- dependency-name: tox
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

pyproject.toml

@@ -67,7 +67,7 @@ xbrl_extract = "ferc_xbrl_extractor.cli:main"
 dev = [
     "build>=1.0,<1.3",
     "ruff>=0.5.0,<0.7", # A very fast linter and autofixer
-    "tox>=4.16.0,<4.19", # Python test environment manager
+    "tox>=4.16.0,<4.21", # Python test environment manager
     "twine>=3.3,<5.2", # Used to make releases to PyPI
 ]
 docs = [
@@ -88,7 +88,7 @@ tests = [
     "pytest-cov>=2.10,<5.1", # Pytest plugin for working with coverage
     "pytest-mock>=3.0,<3.15",
     "ruff>=0.5.0,<0.7", # A very fast linter and autofixer
-    "tox>=4.16.0,<4.19", # Python test environment manager
+    "tox>=4.16.0,<4.21", # Python test environment manager
 ]
 types = ["types-setuptools"]