Move src/tablib/packages/dbfpy to src/tablib/_vendor/dbfpy

[hugovk]

Follow on from #569.

dbfpy is a vendored package. Move it to a _vendor directory to make this clear.

This will be released in tablib 4.0.0.

[codecov]

Codecov Report

Merging #570 (c86b62b) into master (8e2004c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #570   +/-   ##
=======================================
  Coverage   92.40%   92.40%           
=======================================
  Files          28       28           
  Lines        2938     2938           
=======================================
  Hits         2715     2715           
  Misses        223      223           

Files	Coverage Δ
src/tablib/_vendor/dbfpy/dbf.py	68.75% <ø> (ø)
src/tablib/_vendor/dbfpy/dbfnew.py	97.67% <ø> (ø)
src/tablib/_vendor/dbfpy/fields.py	71.27% <ø> (ø)
src/tablib/_vendor/dbfpy/header.py	83.16% <ø> (ø)
src/tablib/_vendor/dbfpy/record.py	76.38% <ø> (ø)
src/tablib/_vendor/dbfpy/utils.py	90.32% <ø> (ø)
src/tablib/formats/_dbf.py	100.00% <100.00%> (ø)
tests/test_tablib_dbfpy_packages_fields.py	100.00% <100.00%> (ø)
tests/test_tablib_dbfpy_packages_utils.py	100.00% <100.00%> (ø)

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[claudep]

Thanks!