Upgrade chrono to 0.4.20 to address RUSTSEC-2020-0159

See the release announcement:

- https://github.com/chronotope/chrono/releases/tag/v0.4.20

It looks like the fix for RUSTSEC-2020-0159 vendors much of the relevant
code from `tz-rs` (which Artichoke already uses):

- chronotope/chrono#677

Previous `cargo deny` error (I think this started triggering today now
that there is a fixed version out):

```console
$ cargo deny check
error[A001]: Potential segfault in `localtime_r` invocations
   ┌─ /Users/lopopolo/dev/artichoke/artichoke/Cargo.lock:15:1
   │
15 │ chrono 0.4.19 registry+https://github.com/rust-lang/crates.io-index
   │ ------------------------------------------------------------------- security vulnerability detected
   │
   = ID: RUSTSEC-2020-0159
   = Advisory: https://rustsec.org/advisories/RUSTSEC-2020-0159
   = ### Impact

     Unix-like operating systems may segfault due to dereferencing a dangling pointer in specific circumstances. This requires an environment variable to be set in a different thread than the affected functions. This may occur without the user's knowledge, notably in a third-party library.

     ### Workarounds

     No workarounds are known.

     ### References

     - [time-rs/time#293](time-rs/time#293)
   = Announcement: chronotope/chrono#499
   = Solution: Upgrade to >=0.4.20
   = chrono v0.4.19
     ├── chrono-tz v0.6.1
     │   └── spinoso-time v0.5.0
     │       └── artichoke-backend v0.13.0
     │           └── artichoke v0.1.0-pre.0
     └── spinoso-time v0.5.0 (*)

advisories FAILED, bans ok, licenses ok, sources ok
```

spinoso-time/Cargo.toml

@@ -20,7 +20,7 @@ tzrs = ["dep:once_cell", "dep:regex", "dep:tz-rs", "dep:tzdb"]
 tzrs-local = ["tzrs", "tzdb?/local"]
 
 [dependencies]
-chrono = { version = "0.4.19", default-features = false, features = ["clock"], optional = true }
+chrono = { version = "0.4.20", default-features = false, features = ["clock"], optional = true }
 chrono-tz = { version = "0.6.0", default-features = false, optional = true }
 once_cell = { version = "1.12.0", optional = true }
 regex =  { version = "1.5.5", default-features = false, features = ["std"], optional = true }