Include doctests in the test suite

- use xdoctest to validate all code snippets in docstrings
- add xdoctest to the nox session "test"

poetry.lock

@@ -137,7 +137,7 @@ version = "7.1.2"
 [[package]]
 category = "dev"
 description = "Cross-platform colored terminal text."
-marker = "sys_platform == \"win32\" or platform_system == \"Windows\""
+marker = "sys_platform == \"win32\" or platform_system == \"Windows\" or platform_system == \"Windows\""
 name = "colorama"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
@@ -885,8 +885,32 @@ optional = false
 python-versions = "*"
 version = "1.12.1"
 
+[[package]]
+category = "dev"
+description = "A rewrite of the builtin doctest module"
+name = "xdoctest"
+optional = false
+python-versions = "*"
+version = "0.13.0"
+
+[package.dependencies]
+six = "*"
+
+[package.dependencies.Pygments]
+optional = true
+version = "*"
+
+[package.dependencies.colorama]
+optional = true
+version = "*"
+
+[package.extras]
+all = ["six", "pytest", "pytest-cov", "codecov", "scikit-build", "cmake", "ninja", "pybind11", "pygments", "colorama"]
+optional = ["pygments", "colorama"]
+tests = ["pytest", "pytest-cov", "codecov", "scikit-build", "cmake", "ninja", "pybind11"]
+
 [metadata]
-content-hash = "899f376a54c187e41392fb83831a59926668b662bfb32004f4a0964c1202698c"
+content-hash = "79e46e67260312ba1b227f7717d0ba781a6ad5df352f83fe6e1b72a98ad1ec5b"
 lock-version = "1.0"
 python-versions = "^3.8"
 
@@ -1311,3 +1335,7 @@ wemake-python-styleguide = [
 wrapt = [
     {file = "wrapt-1.12.1.tar.gz", hash = "sha256:b62ffa81fb85f4332a4f609cab4ac40709470da05643a082ec1eb88e6d9b97d7"},
 ]
+xdoctest = [
+    {file = "xdoctest-0.13.0-py2.py3-none-any.whl", hash = "sha256:de861fd5230a46bd26c054b4981169dd963f813768cb62b62e104e4d2644ac94"},
+    {file = "xdoctest-0.13.0.tar.gz", hash = "sha256:4f113a430076561a9d7f31af65b5d5acda62ee06b05cb6894264cb9efb8196ac"},
+]