Remove --mpl from pytest config; options add missing dependency

* `topofileformats/asd.py` imports `matplotlib.pyplot` but `matplotlib` isn't a dependency listed in `pyproject.toml`
* Removes `--mpl` as an option to `pytest` configuration which is in `pyproject.toml` under the
  `[tool.pytest.ini_options]` section.

pyproject.toml

@@ -22,7 +22,10 @@ keywords = [
   "afm",
   "image processing"
 ]
-dependencies = ["numpy"]
+dependencies = [
+  "matplotlib",
+  "numpy"
+]
 
 [project.optional-dependencies]
 dev = [
@@ -39,7 +42,7 @@ dev = [
 
 [tool.pytest.ini_options]
 minversion = "7.0"
-addopts = "--cov --mpl"
+addopts = "--cov"
 testpaths = [
     "tests",
 ]