MAINT: drop support for python 3.6

[jklenzing]

Description

The operational requirement for python 3.6 has been cycled to python 3.9. A number of old try / except statements can be dropped to simplify the code and remove backwards compatibility. A minimum of python 3.9 should be implemented.

Potential impact

• Is the feature related to an existing problem?
  • We currently have no way of testing python 3.6. Without a requirement to support this, maintaining backwards compatibility is a timesink.
• How critical is this feature to your workflow?
  • Simplify testing moving forward.
• How wide of an impact to you anticipate this enhancement having?
  • Minimal, as new python 3.6 environments are unlikely. Currently cannot install anything older than python 3.7 on my desktop.
• Would this break any existing functionality?
  • It would break backwards compatibility.

Potential solution(s)

• Add minimum of python 3.9
• Remove if / else and try / except related to this version
• Remove support for cdflib < 1.0

Alternatives

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Additional context

Testing environments updated for v0.0.6. Code left the same as this is a significant rewrite.