Remove dependency on dev callr

Author: lionel-

DESCRIPTION

@@ -24,7 +24,7 @@ Imports:
     R6,
     utils
 Suggests:
-    callr (>= 3.7.3.9001),
+    callr (>= 3.7.3),
     cli (>= 3.3.0),
     codetools,
     covr,

tests/testthat/test-process.R

@@ -69,6 +69,9 @@ test_that("working directory does not exist", {
 })
 
 test_that("R process is installed with a SIGTERM cleanup handler", {
+  # https://github.com/r-lib/callr/pull/250
+  skip_if_not_installed("callr", "3.7.3.9001")
+
   # Needs POSIX signal handling
   skip_on_os("windows")