Set new version number to 4.0.0

[micafer]

Set new version number to 4.0.0

Description

Set new version number to 4.0.0, as one of the changes of the new release can be backward-incompatible.

Status

Replace this: describe the PR status. Examples:

• done, ready for review

Checklist (tick everything that applies)

• Code linting (required, can be done after the PR checks)
• Documentation
• Tests
• ICLA (required for bigger changes)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.58%. Comparing base (1e66b48) to head (4893b91).

Additional details and impacted files

@@ Coverage Diff @@## trunk #2178 +/- ##
=======================================
Coverage 83.58% 83.58% =======================================
Files 352 352 Lines 81808 81808 Branches 8769 8769 =======================================
Hits 68373 68373 Misses 10562 10562 Partials 2873 2873 

Files with missing lines	Coverage Δ
libcloud/__init__.py	68.29% <100.00%> (ø)

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