Add daqmx 21.5 support

[zhindes]

Overview

This adds support for DAQmx 21.5 API elements to nidaqmx-python along with additional improvements.

• No more empty docstrings on constants.
• Fix C API function mapping for attributes - dozens were incorrect.
• Remove some internal-only enumerations that were unused.
• (compat breaker) Fix two egregious naming issues when translating the API to SNAKE_CASE. M_HZ is now MHZ (megahertz) and <word>m_VOLTS is now MILLIVOLTS.
• (compat breaker) Fix various constant names that didn't make any sense.
• Add a header to all auto-generated files indicating that they should not be edited by hand.
• Closes __future__ imports are now all mandatory in the minimum supported python version #132

Testing

All tests pass:

  py37: commands succeeded
  py38: commands succeeded
  py39: commands succeeded
  py310: commands succeeded
  congratulations :)

[zhindes]

Need to update CONTRIBUTING.md

[zhindes]

Need to update CONTRIBUTING.md

Done!