Update askama requirement from 0.11.0 to 0.12.0 (#610)

Updates the requirements on [askama](https://github.com/djc/askama) to permit the latest version.
- [Release notes](https://github.com/djc/askama/releases)
- [Commits](djc/askama@0.11.0...0.12.0)

---
updated-dependencies:
- dependency-name: askama
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

examples/templating/askama/Cargo.toml

@@ -6,4 +6,4 @@ edition = "2018"
 
 [dependencies]
 gotham = { path = "../../../gotham" }
-askama = { version = "0.11.0", default-features = false }
+askama = { version = "0.12.0", default-features = false }