Rename _asyncio to _io

[jacobtomlinson]

To support #77 this PR renames the _asyncio submodule containing asyncio helper tools to _io to communicate that we will support more than just asyncio.

[codecov]

Codecov Report

Merging #85 (1fe3601) into main (8826a6f) will not change coverage.
The diff coverage is 95.45%.

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  Coverage   96.84%   96.84%           
=======================================
  Files          24       23    -1     
  Lines        1903     1903           
=======================================
  Hits         1843     1843           
  Misses         60       60           

Impacted Files	Coverage Δ
kr8s/_io.py	92.63% <ø> (ø)
kr8s/tests/test_io.py	94.11% <94.11%> (ø)
kr8s/__init__.py	100.00% <100.00%> (ø)
kr8s/_auth.py	91.20% <100.00%> (ø)
kr8s/objects.py	100.00% <100.00%> (ø)
kr8s/portforward.py	100.00% <100.00%> (ø)