tests: centralize --doctest-modules option in pytest.ini

We have meaningful doctests in the code base. Before this change, their
execution had to be manually activated in Makefile and tox.ini.

This change centralizes the option in pytest.ini, so that we reduce the future
occurrence of human errors (less duplication, less oversights).

Makefile

@@ -154,7 +154,7 @@ darglint-files: ## run darglint for specific files (specified with files="file1
 test: ## run tests quickly with the default Python
 	pytest                              \
 		tests/                          \
-		--doctest-modules black_it      \
+		black_it                        \
 		--cov=black_it                  \
 		--cov-report=xml                \
 		--cov-report=html               \

pytest.ini

@@ -6,3 +6,5 @@ log_cli_date_format=%Y-%m-%d %H:%M:%S
 
 markers =
     e2e: marks end-to-end tests which involve several library components.
+
+addopts = --doctest-modules

tox.ini

@@ -21,7 +21,7 @@ deps =
     mistletoe==1.2.1
 
 commands =
-    pytest --basetemp={envtmpdir} --doctest-modules \
+    pytest --basetemp={envtmpdir} \
         black_it tests/ \
         --cov=black_it \
         --ignore=example/models \