change: add class to read Python scripts and update code for v2

[laurenyu]

Issue #, if available:
#1478

Description of changes:
follow-up to #1492. This class reads in Python source files and feeds the code to ASTTransformer.

unit tests will come later (see #1478)

Testing done:
manual testing:

# sample input script
$ cat input.py
from sagemaker.mxnet import MXNet

m = MXNet()  # TODO: fill in parameters
m.fit()

# run with an output_path that creates directories
$ python -c "import files; files.PyFileUpdater('input.py', 'foo/bar/output.py').update()" && cat foo/bar/output.py
from sagemaker.mxnet import MXNet

m = MXNet(framework_version='1.2.0')  # TODO: fill in parameters
m.fit()

# run with an output_path that doesn't contain directories
$ python -c "import files; files.PyFileUpdater('input.py', 'output.py').update()" && cat output.py
from sagemaker.mxnet import MXNet

m = MXNet(framework_version='1.2.0')  # TODO: fill in parameters
m.fit()

# run again to check for warning message from the output file already existing
$ python -c "import files; files.PyFileUpdater('input.py', 'output.py').update()" && cat output.py
Overwriting file output.py
from sagemaker.mxnet import MXNet

m = MXNet(framework_version='1.2.0')  # TODO: fill in parameters
m.fit()

# run using the same path for both input_file and output_file
$ python -c "import files; files.PyFileUpdater('input.py', 'input.py').update()" && cat input.py
Overwriting file input.py
from sagemaker.mxnet import MXNet

m = MXNet(framework_version='1.2.0')  # TODO: fill in parameters
m.fit()
(venv) ~/Documents/workspace/sagemaker-python-sdk/tools/compatibility/v2$

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