Explicit Python >= 3.7 support

[leycec]

The PyPI project page for pymerkletools currently only lists explicit supports for Python <= 3.6. More importantly, TravisCI tests currently only test pymerkletools under Python 2.7 and 3.5. However, I can personally confirm that pymerkletools both installs and behaves as expected under at least Python 3.7. The same probably follows for Python 3.8 as well.

Thankfully, the fix appears trivial:

• Explicitly list Python 3.6, 3.7, and 3.8 in .travis.yml to guarantee compatibility: e.g.,

python:
  - "2.7"
  - "3.5"
  - "3.6"
  - "3.7"
  - "3.8"

• Explicitly list Python 3.5, 3.6, 3.7, and 3.8 in setup.py for PyPI readability: e.g.,

    classifiers=[
        "Intended Audience :: Developers",
        "Intended Audience :: Education",
        "Intended Audience :: Science/Research",
        "License :: OSI Approved :: MIT License",
        "Programming Language :: Python :: 2.7",
        "Programming Language :: Python :: 3.5",
        "Programming Language :: Python :: 3.6",
        "Programming Language :: Python :: 3.7",
        "Programming Language :: Python :: 3.8",
        "Topic :: Software Development :: Libraries",
        "Topic :: Software Development :: Libraries :: Python Modules"
    ],

Thanks for all Merkle, @Tierion! You rock the Bitcoin Casbah. :1st_place_medal:

[KuzonFyre]

I am leaving this as a warning to others. I installed an older version of python to run this but was having trouble specifying which python to install it to. Use this command

python3.6 -m pip install merkletools

to specify which python pip is installing merkle tools to.