Set upper bound for Python (databricks#152)

### Description

The dependent library `databricks-sql-connector` doesn't support Python 3.10 yet (databricks/databricks-sql-python#26).
We should also avoid incompatible Python version.

Author: ueshin

.github/workflows/main.yml

@@ -80,7 +80,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.7", "3.8", "3.9"]
 
     env:
       TOXENV: "unit"
@@ -177,7 +177,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.7", "3.8", "3.9"]
 
     steps:
       - name: Set up Python ${{ matrix.python-version }}

setup.py

@@ -68,7 +68,6 @@ def _get_plugin_version():
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
-        "Programming Language :: Python :: 3.10",
     ],
-    python_requires=">=3.7",
+    python_requires=">=3.7,<3.10",
 )