minor version bump to 1.6.0

[aranke]

Changelog:

• Fix case-insensitive env vars for Windows (Fix case-insensitive env vars for Windows #166)
• Conditionally import deepdiff (conditionally import deepdiff #164)
• Improve Type Annotations Coverage (Improve Type Annotations Coverage #162)
• add to and to_column(s) to ColumnLevelConstraint and ModelLevelConstraint contracts (add to and to_column(s) to ColumnLevelConstraint and ModelLevelConstraint contracts #163)

[github-actions]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 64.31%. Comparing base (8edae92) to head (1316c81).

Files	Patch %	Lines
dbt_common/__about__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #167   +/-   ##
=======================================
  Coverage   64.31%   64.31%           
=======================================
  Files          51       51           
  Lines        3225     3225           
=======================================
  Hits         2074     2074           
  Misses       1151     1151           

Flag	Coverage Δ
unit	64.31% <0.00%> (ø)

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[MichelleArk]

Let's add #163 to the changelog once its in but otherwise LGTM!