build: pyproject.toml allows Python 3.6; remove deps from toml, use requirements.txt

Author: chaimleib

pyproject.toml

@@ -6,10 +6,7 @@ authors = [
   { name="Chaim Leib Halbert", email="[email protected]" },
 ]
 readme = "README.md"
-requires-python = ">=3.7"
-dependencies = [
-	"sortedcontainers < 3",
-]
+requires-python = ">=3.6"
 classifiers = [
         'Development Status :: 5 - Production/Stable',
         'Programming Language :: Python :: Implementation :: PyPy',
@@ -18,6 +15,7 @@ classifiers = [
         'Intended Audience :: Science/Research',
         'Programming Language :: Python',
         'Programming Language :: Python :: 3',
+        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',