Simplify version and imports

[DimitriPapadopoulos]

Move all imports at the top as expected. Initialize __version__ after imports.

Move _is_file() near other function definitions, after constant definitions.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.45%. Comparing base (315f6c1) to head (8e935a6).
⚠️ Report is 20 commits behind head on master.

Files with missing lines	Patch %	Lines
tabulate/__init__.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #415   +/-   ##
=======================================
  Coverage   84.45%   84.45%           
=======================================
  Files           1        1           
  Lines         907      907           
  Branches      218      218           
=======================================
  Hits          766      766           
  Misses        107      107           
  Partials       34       34           

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