build(deps): Bump syn from 2.0.70 to 2.0.71 (#571)

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.70 to 2.0.71.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/dtolnay/syn/releases">syn's
releases</a>.</em></p>
<blockquote>
<h2>2.0.71</h2>
<ul>
<li>Do not require mutable borrow in Punctuated::get() (<a
href="https://redirect.github.com/dtolnay/syn/issues/1706">#1706</a>,
thanks <a
href="https://github.com/lemunozm"><code>@​lemunozm</code></a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/dtolnay/syn/commit/f34dc7bb53185a11f3aa45fc691488547c8c76ee"><code>f34dc7b</code></a>
Release 2.0.71</li>
<li><a
href="https://github.com/dtolnay/syn/commit/896d58bdb2508a089100247f01f7271222c518df"><code>896d58b</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1706">#1706</a>
from lemunozm/patch-1</li>
<li><a
href="https://github.com/dtolnay/syn/commit/dd7e269f577dc33bcf92bec2583f7e53609081fb"><code>dd7e269</code></a>
Remove mut from Puntuated::get()</li>
<li>See full diff in <a
href="https://github.com/dtolnay/syn/compare/2.0.70...2.0.71">compare
view</a></li>
</ul>
</details>
<br />


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