test: run doctest on rst files in docs/ (#1875)

Currently there are only two files with doctests inside, but it is best
that we always run doctests just to ensure things are sane.

Other changes:

- added `doctest: +SKIP` to make doctests inside `docs/rdf_terms.rst`
  pass.

Fixes #1791

docs/rdf_terms.rst

@@ -106,7 +106,7 @@ Here is the *URIRef* class' auto-built documentation:
 .. code-block:: python
 
     >>> from rdflib import URIRef
-    >>> uri = URIRef()
+    >>> uri = URIRef()  # doctest: +SKIP
     Traceback (most recent call last):
       File "<stdin>", line 1, in <module>
     TypeError: __new__() missing 1 required positional argument: 'value'
@@ -137,9 +137,9 @@ Here is the *BNode* class' auto-built documentation:
 
     >>> from rdflib import BNode
     >>> bn = BNode()
-    >>> bn
+    >>> bn  # doctest: +SKIP
     rdflib.term.BNode('AFwALAKU0')
-    >>> bn.n3()
+    >>> bn.n3()  # doctest: +SKIP
     '_:AFwALAKU0'
     
     

pyproject.toml

@@ -35,6 +35,7 @@ addopts = [
    "--ignore=admin",
    "--ignore=rdflib/extras/external_graph_libs.py",
    "--ignore-glob=docs/*.py",
+   "--doctest-glob=docs/*.rst",
 ]
 doctest_optionflags = "ALLOW_UNICODE"
 filterwarnings = [