Contributing.md: Minor pytest-related edits

* URL-ise some things.
* `` around some code.

Contributing.md

@@ -164,7 +164,7 @@ Where possible please write unit tests for your PRs, especially in the case of
 bug fixes, having regression tests help ensure that we don't accidentally
 re-introduce a bug down the line.
 
-We use the `pytest` for unit testing.
+We use the [`pytest`](https://docs.pytest.org/en/stable/) for unit testing.
 
 The files for a test should go in a sub-directory of `tests/` named after the
 (principal) file that contains the code they are testing.  e.g. for
@@ -178,15 +178,15 @@ containing test data.
 Invoking just a bare `pytest` command will run all tests.
 
 To run only a sub-set of tests you can use, e.g. `pytest -k journal_lock`. You
-might want to use `pytest -rA -k journal_lock` if you have any debug print()
+might want to use `pytest -rA -k journal_lock` if you have any debug `print()`
 statements within the test code itself, so you can see the output even when the
 tests all succeed.
 
-Adding `--trace` to a `pytest` invocation causes it to drop into `pdb` prompt
-for each test, handy if you want to step through the testing code to be sure of
-anything.
+Adding `--trace` to a `pytest` invocation causes it to drop into a
+[`pdb`](https://docs.python.org/3/library/pdb.html) prompt for each test,
+handy if you want to step through the testing code to be sure of anything.
 
-Otherwise see the [pytest documentation](https://docs.pytest.org/en/stable/contents.html).
+Otherwise, see the [pytest documentation](https://docs.pytest.org/en/stable/contents.html).
 
 ---