update dependency versions

Author: matthewhanson

setup.py

@@ -27,10 +27,10 @@
     packages=find_packages(),
     py_modules=[splitext(basename(path))[0] for path in glob('pystac_client/*.py')],
     include_package_data=False,
+    python_requires=">=3.7",
     install_requires=[
-        "python-dateutil>=2.7.0",
-        "requests~=2.25",
-        "pystac~=1.1.0"
+        "requests>=2.25",
+        "pystac~=1.2.0"
     ],
     extras_require={
         "validation": ["jsonschema==3.2.0"]
@@ -47,10 +47,10 @@
     classifiers=[
         "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",
+        "Programming Language :: Python :: 3.10",
         "Operating System :: OS Independent",
         "Natural Language :: English",
         "Development Status :: 3 - Alpha",