chore(deps): pin dependency node-fetch to 2.6.1

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
node-fetch	devDependencies	pin	^2.6.1 -> 2.6.1

📌 Important: Renovate will wait until you have merged this Pin PR before creating any upgrade PRs for the affected packages. Add the preset :preserveSemverRanges to your config if you instead don't wish to pin dependencies.

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[coveralls]

Coverage remained the same at 89.796% when pulling cf435dd on renovate/pin-dependencies into 6090c00 on master.