jpwr compatibility with Python version <3.9

[chelseajohn]

When using jpwr with python versions < 3.9

Traceback (most recent call last):
  File "/p/project1/jscbenchmark/john2/CARAML/resnet50/resnet50_benchmark_run/000000/000000_execute/work/benchmarks/scripts/tf_cnn_benchmarks/tf_cnn_benchmarks.py", line 73, in <module>
    app.run(main)  # Raises error on invalid flags, unlike tf.app.run()
  File "/usr/local/lib/python3.8/dist-packages/absl/app.py", line 308, in run
    _run_main(main, args)
  File "/usr/local/lib/python3.8/dist-packages/absl/app.py", line 254, in _run_main
    sys.exit(main(argv))
  File "/p/project1/jscbenchmark/john2/CARAML/resnet50/resnet50_benchmark_run/000000/000000_execute/work/benchmarks/scripts/tf_cnn_benchmarks/tf_cnn_benchmarks.py", line 68, in main
    bench.run()
  File "/p/project1/jscbenchmark/john2/CARAML/resnet50/resnet50_benchmark_run/000000/000000_execute/work/benchmarks/scripts/tf_cnn_benchmarks/benchmark_cnn.py", line 1906, in run
    return self._benchmark_train()
  File "/p/project1/jscbenchmark/john2/CARAML/resnet50/resnet50_benchmark_run/000000/000000_execute/work/benchmarks/scripts/tf_cnn_benchmarks/benchmark_cnn.py", line 2111, in _benchmark_train
    return self._benchmark_graph(result_to_benchmark, eval_build_results)
  File "/p/project1/jscbenchmark/john2/CARAML/resnet50/resnet50_benchmark_run/000000/000000_execute/work/benchmarks/scripts/tf_cnn_benchmarks/benchmark_cnn.py", line 2316, in _benchmark_graph
    stats = self.benchmark_with_session(
  File "/p/project1/jscbenchmark/john2/CARAML/resnet50/resnet50_benchmark_run/000000/000000_execute/work/benchmarks/scripts/tf_cnn_benchmarks/benchmark_cnn.py", line 2430, in benchmark_with_session
    from jpwr.gpu.pynvml import power
  File "/p/project1/jscbenchmark/john2/CARAML/resnet50/nvidia_arm_tensorflow_packages/lib/python3.8/site-packages/jpwr/gpu/pynvml.py", line 6, in <module>
    class power(object):
  File "/p/project1/jscbenchmark/john2/CARAML/resnet50/nvidia_arm_tensorflow_packages/lib/python3.8/site-packages/jpwr/gpu/pynvml.py", line 7, in power
    def init(self, power_value_dict : dict[str,list[float]]):
TypeError: 'type' object is not subscriptable

Solution to make annotations compatible with Python <3.9
Include from __future__ import annotations line at the beginning of the file.