fix: enforce coverage<6 and ensure python 3.10 support

coverage.py v6+ introduces breaking changes which affect
readme-coverage-badger

setup.cfg

@@ -23,6 +23,7 @@ classifiers =
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
     Topic :: Software Development :: Testing
     Topic :: Text Processing :: Markup :: Markdown
     Topic :: Utilities
@@ -39,7 +40,7 @@ include_package_data = True
 python_requires = >=3.6
 install_requires =
     colorama
-    coverage
+    coverage <6
 
 [options.entry_points]
 console_scripts =
@@ -49,7 +50,7 @@ console_scripts =
 
 [flake8]
 ignore = E203, E266, E501, W503, F403, F401
-max-line-length = 88
+max-line-length = 120
 max-complexity = 18
 select = B,C,E,F,W,T4,B9
 exclude = .git,__pycache__,__init__.py,.mypy_cache,.pytest_cache,.tox
@@ -93,11 +94,12 @@ testpaths = tests
 # --- tox automation configuration -------------------------------------------
 
 [tox:tox]
-envlist = lint, py36, py37, py38, py39
+envlist = lint, py36, py37, py38, py39, py310
 isolated_build = True
 
 [gh-actions]
 python =
+    3.10: py310
     3.9: py39
     3.8: lint, py38
     3.7: py37