Add support for Python 3.10

[hugovk]

Python 3.10 was released last week:

• https://discuss.python.org/t/python-3-10-0-is-now-available/10955

Only test 3.10 on Ubuntu for now, because binary wheels for pandas on macOS and Windows are not yet available and the build will fail.

[codecov[bot]]

Codecov Report

Merging #504 (509682a) into master (37ce048) will increase coverage by 0.02%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
+ Coverage   90.78%   90.80%   +0.02%     
==========================================
  Files          28       28              
  Lines        2647     2653       +6     
==========================================
+ Hits         2403     2409       +6     
  Misses        244      244              

Impacted Files	Coverage Δ
src/tablib/packages/dbfpy/dbfnew.py	97.67% <0.00%> (+0.05%)	:arrow_up:
src/tablib/packages/dbfpy/fields.py	71.27% <0.00%> (+0.15%)	:arrow_up:
src/tablib/packages/dbfpy/utils.py	90.16% <0.00%> (+0.16%)	:arrow_up:
src/tablib/packages/dbfpy/header.py	83.83% <0.00%> (+0.16%)	:arrow_up:
src/tablib/packages/dbfpy/dbf.py	68.75% <0.00%> (+0.32%)	:arrow_up:
src/tablib/packages/dbfpy/record.py	76.05% <0.00%> (+0.34%)	:arrow_up:

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