build: update devdependency lerna to ~3.21.0

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
lerna	devDependencies	minor	~3.20.0 -> ~3.21.0

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Release Notes

lerna/lerna

v3.21.0

Compare Source

Bug Fixes

• publish: Canary releases without previous tags should not explode (c9eb590)

Features

• project: Add getPackagesSync() export (068bdd7)
• version: Ignore private packages completely with --no-private (a9b9f97)

3.20.2 (2020-01-02)

Bug Fixes

• version: Loosen --atomic fallback to catch incompatible CLI versions (6f0e2bb), closes #​2400

3.20.1 (2019-12-29)

Bug Fixes

• version: Support git clients that do not support git push --atomic (2b9b210)

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