fix doc link/add py2

Author: joknarf

pyproject.toml

@@ -22,6 +22,8 @@ classifiers = [
   "Operating System :: Unix",
   "Operating System :: MacOS",
   "Programming Language :: Python",
+  "Programming Language :: Python :: 2",
+  "Programming Language :: Python :: 2.7",
   "Programming Language :: Python :: 3",
   "Programming Language :: Python :: 3.6",
   "Programming Language :: Python :: 3.7",
@@ -38,7 +40,7 @@ classifiers = [
 
 [project.urls]
 Homepage = "https://github.com/joknarf/pgtree"
-Documentation = "https://github.com/joknarf/pgtree/blob/main/README.md"
+Documentation = "https://github.com/joknarf/pgtree/blob/master/README.md"
 Repository = "https://github.com/joknarf/pgtree.git"
 
 [build-system]