[tests] Add README.rst check

Neither PyPi nor GitHub support the sphinx-specific directives,
obviously. So adding a check in tox to ensure the restructuredtext
is clean.

README.rst

@@ -132,7 +132,7 @@ Simple, right ? And you can still use it like a normal function, typically when
 
     f(Database())
 
-:py:func:`.inject` here used the marker :code:`inject.me()` with the help of the type hint to determine
+:code:`.inject` here used the marker :code:`inject.me()` with the help of the type hint to determine
 the dependency. But it also supports the following ways to express the dependency wiring:
 
 - annotated type hints:

tox.ini

@@ -1,13 +1,13 @@
 [tox]
 ; need no_attrs instead of nothing for tox-travis (see travis:env)
-envlist = manifest,flake8,mypy,py{37,38,39,310}{,-compiled},coverage-report,docs
+envlist = manifest,flake8,mypy,py{37,38,39,310}{,-compiled},coverage-report,docs,rstvalidator
 isolated_build = true
 
 [gh-actions]
 python =
     3.7: py37{,-compiled}
     3.8: py38{,-compiled}
-    3.9: py39{,-compiled},flake8,manifest,docs,pyright
+    3.9: py39{,-compiled},flake8,manifest,docs,pyright,rstvalidator
     3.10: py310{,-compiled}
 
 [testenv]
@@ -69,6 +69,16 @@ max-line-length = 100
 ; max-complexity = 10
 
 
+[testenv:rstvalidator]
+changedir = {toxinidir}
+skip_install = true
+deps =
+    rstvalidator==2020.12.3
+    pygments==2.11.2
+commands =
+    python -m rstvalidator README.rst
+
+
 [testenv:docs]
 changedir = docs
 deps =