Cleanup root (#221)

* delete unused requirements.txt

* delete unused setup.py (see more)

As per sunpy docs (https://docs.sunpy.org/projects/package-template/en/latest/minimal.html#setup-py),

"It is no longer necessary to include a setup.py file in your project, unless you are building C extensions in your code. However, it can increase compatibility with old versions of pip and other packaging tools."

* pyproject: add dev target

* changelog

Author: settwi

changelog/221.trivial.rst

@@ -0,0 +1 @@
+Remove unused files from root directory; add `dev` target to pyproject

pyproject.toml

@@ -48,6 +48,10 @@ docs = [
   "sphinx-changelog",
   "sphinx-gallery",
 ]
+dev = [
+    "sunkit-spex[docs,tests]",
+    "tox"
+]
 
 [project.urls]
 Homepage = "https://sunkit-spex.readthedocs.io"