Ensure invoke lint task & its docs are in sync

Someday the entire Pelican code base will perhaps have Black and isort
formatting applied to it, but it doesn't make sense for the linter to
check that until it's actually done. This changes the `lint` Invoke task
to run Flake8 on changed lines only, which should bring it into line
with both the current Pre-commit and CI configurations.

Also, the docs erroneously stated that `invoke tests` would also check
for code style compliance, which has herein been fixed.

docs/contribute.rst

@@ -75,11 +75,14 @@ via::
 
     invoke tests
 
-In addition to running the test suite, the above invocation will also check code
-style and let you know whether non-conforming patterns were found. In some cases
-these linters will make the needed changes directly, while in other cases you
-may need to make additional changes until ``invoke tests`` no longer reports any
-code style violations.
+In addition to running the test suite, it is important to also ensure that any
+lines you changed conform to code style guidelines. You can check that via::
+
+    invoke lint
+
+If code style violations are found in lines you changed, correct those lines
+and re-run the above lint command until they have all been fixed. You do not
+need to address style violations, if any, for code lines you did not touch.
 
 After making your changes and running the tests, you may see a test failure
 mentioning that "some generated files differ from the expected functional tests

tasks.py

@@ -70,13 +70,11 @@ def isort(c, check=False, diff=False):
 
 @task
 def flake8(c):
-    c.run(f"{VENV_BIN}/flake8 {PKG_PATH} tasks.py")
+    c.run(f"git diff HEAD | {VENV_BIN}/flake8 --diff --max-line-length=88")
 
 
 @task
 def lint(c):
-    isort(c, check=True)
-    black(c, check=True)
     flake8(c)