Specify and install Python dependencies in a requirements.txt file

[brettcannon]

Right now we have some Python source code checked into our repository. We should move away from that and over to pinned versions that we install via pip.

The install command should be:

python -m pip install --target pythonFiles --implementation py --no-deps -r requirements.txt

The requirements.txt files should be:

# Needs 'futures' for Python 2; see https://github.com/Microsoft/vscode-python/issues/2022 .
isort==4.3.4
parso==0.2.1
jedi==0.12.0
# Should probably start pinning ptvsd.
ptvsd

And with that we can drop almost all of the third-party code in pythonFiles that we have checked into our repo. This will require updating where we install ptvsd, though, as well as probably renaming/moving the requirements.txt file we already have in the top-level directory that specifies requirements for tests.

[DonJayamanne]

Should probably start pinning ptvsd.

However for CI testing purposes, I'd like to ensure we have the latest Master branch of PTVSD.

[brettcannon]

@DonJayamanne I knew you were going to say that 😉 . That's fine, but for general development and especially for building the extension we shouldn't do that.

[DonJayamanne]

Just wanted to ensure we do not forget about that requirement (for testing purposes), either way, I do agree with the suggestion.