[WIP] Update to latest Alpine and pip packages

[robb-romans]

• Continues from Dependencies upgrade #76
• Closes Update package dependencies in requirements.txt #73
• Alpine 3.8
• Pip is now provided in base python3 package
• Placeholder to remove build deps for security and size
  (needs runtime testing)
• Use new Python3 venv builtin (pyvenv is deprecated)
• MAINTAINER is deprecated
• Use latest pip packages in requirements
• Update gitignore

[robb-romans]

currently stuck on entry_point problem

Running Sphinx v1.7.8
Traceback (most recent call last):
  File "/venv/lib/python3.6/site-packages/sphinx/registry.py", line 87, in preload_builder
    entry_point = next(entry_points)
StopIteration

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/preparer/deconstrst/__main__.py", line 5, in <module>
    main()
  File "/preparer/deconstrst/__init__.py", line 44, in main
    status = build(srcdir, destdir)
  File "/preparer/deconstrst/deconstrst.py", line 32, in build
    parallel=1)
  File "/venv/lib/python3.6/site-packages/sphinx/application.py", line 194, in __init__
    self.preload_builder(buildername)
  File "/venv/lib/python3.6/site-packages/sphinx/application.py", line 297, in preload_builder
    self.registry.preload_builder(self, name)
  File "/venv/lib/python3.6/site-packages/sphinx/registry.py", line 90, in preload_builder
    ' through entry point') % name)
sphinx.errors.SphinxError: Builder name deconst-serial not registered or available through entry point