Prepare 7.1.0

[jonasfj]

Would be nice if we can release this, all previous versions of markdown 7.x has issues. It's possible we should release a 7.0.3 cherry picking:

• 5f98aea7e51cb1508d335e23edb2141d5102ce29
• 86ebc2c9b33ca18e86fa69c4154505ab576c6a38

But that's also a bit overkill. It's probably fine to get the next thing out too.

Fixes https://github.com/dart-lang/markdown/issues/537

[coveralls]

Pull Request Test Coverage Report for Build 4860602078

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.374%

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Totals
Change from base Build 4848074552:	0.0%
Covered Lines:	1515
Relevant Lines:	1572

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💛 - Coveralls