Use setuptools-scm to ensure all repo files are in sdist (#235)

Listing `setuptools_scm` in `pyproject.toml` enables it to be used to
determine which files are included in the sdist instead of the older
`MANIFEST.in` procedure. `setuptools_scm` just includes every file
tracked by `git` which seems fine for this small project.

Additionally, delete `INSTALL.txt` which had outdated installation
information.

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["setuptools"]
+requires = ["setuptools>=64", "setuptools-scm>=8"]
 build-backend = "setuptools.build_meta"
 
 [project]
@@ -52,6 +52,8 @@ dependencies = []
 [tool.setuptools]
 packages = ["uncertainties", "uncertainties.unumpy"]
 
+[tool.setuptools_scm]
+
 [project.urls]
 Documentation = "http://uncertainties-python-package.readthedocs.io/"
 Repository = "https://github.com/lmfit/uncertainties"