Relax the tempfile requirement, and lock it to 3.6.0 during CI

To fix the build on the MSRV.  The build will fail if anybody tries to
use 3.7.0 (with any rustc version), but 3.7.1 is ok with newer
toolchains.

Author: asomers

Cargo.lock.msrv

@@ -82,7 +82,7 @@ assert-impl = "0.1"
 lazy_static = "1.4"
 parking_lot = "0.12"
 rand = "0.8"
-tempfile = "3.7.1"
+tempfile = "3.3"
 semver = "1.0.7"
 
 [target.'cfg(any(target_os = "android", target_os = "linux"))'.dev-dependencies]