Updated python package requirements.

[juliettejames]

Updated python package requirements for support of Python 3.7, 3.8 and 3.9.
Updated Read.me to reflect new changes.

[zzhengnan]

Hi @juliettejames and @mhallsmoore. Is the plan to stop supporting conda going forward?

Some additional context

• I helped create a conda package for qstrader in Create conda package #356
• When the 0.2.3 release came out, I got pinged in this auto-created PR qstrader v0.2.3 conda-forge/qstrader-feedstock#2
• I then traced my way back here and realized conda is no longer mentioned in the README

It's perfectly fine with me if we decide not to publish via conda anymore, but if it's extra manual work you are worried about, conda actually has a robust automation system. To name a few

• Whenever a new release comes out on PyPI, a new PR (like the one above) will be opened automatically in the conda repo
• Upon merging the PR, a new conda package matching the PyPI package will be uploaded automatically

I'm also more than happy to add both of you to the recipe maintainer list. Please let me know either way. Thanks!