deps: enable urllib3 2.x on Py>=3.9; keep Py<3.9 on urllib3<2

- Add env-marked pins:
  - Py>=3.9: requests>=2.32.2, urllib3!=2.0.0,<3
  - 3.7–3.8: requests<2.32, urllib3<2
  - 3.6: requests<2.28, urllib3<2
- Bump requests-toolbelt to >=1.0.0
- README: document per-Python resolver behavior
- CI: matrix runs both urllib3<2 and urllib3>=2 on appropriate Pythons

Refs:
- Requests now requires Python >=3.9; 2.32.0/2.32.1 yanked (start at 2.32.2).
- urllib3 2.x requires Python >=3.9.
- requests-toolbelt 1.0.0 aligns with urllib3 2.x.

Author: ahnv

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
         runs-on: ubuntu-20.04
         strategy:
             matrix:
-                python: [3.7, 3.8, 3.9, '3.10']
+                python: [3.7, 3.8, 3.9, '3.10', '3.11', '3.12']
         steps:
             - uses: actions/checkout@v1
             - name: Setup Python

requirements/requirements.txt

@@ -1,3 +1,18 @@
-requests>=2.22.0
-requests_toolbelt==0.10.1
-urllib3==1.26.*
+requests_toolbelt>=1.0.0
+
+# ------------ Python >= 3.9 ------------
+# Requests 2.32.0/2.32.1 were yanked; start at 2.32.2.
+# urllib3 2.x requires Python >=3.9; exclude 2.0.0 (known bug).
+requests>=2.32.2; python_version >= "3.9"
+urllib3!=2.0.0,<3; python_version >= "3.9"
+
+# ------------ 3.7 <= Python < 3.9 ------------
+# Latest Requests that still supports 3.7/3.8 is 2.31.x.
+requests>=2.22.0,<2.32; python_version >= "3.7" and python_version < "3.9"
+# urllib3<2 keeps behavior consistent on these Pythons.
+urllib3<2; python_version >= "3.7" and python_version < "3.9"
+
+# ------------ Python < 3.7 (i.e., 3.6) ------------
+# 2.27.x was the last Requests series for 3.6.
+requests>=2.22.0,<2.28; python_version < "3.7"
+urllib3<2; python_version < "3.7"