Spack doesn't need `requests`, and neither does `jsonschema`, but
`jsonschema` tries to import it, and it'll succeed if `requests` is on
your machine (which is likely, given how popular it is). This commit
removes the import to improve Spack's startup time a bit.
On a mac with SSD, the import of requests is ~28% of Spack's startup time
when run as `spack --print-shell-vars sh,modules` (.069 / .25 seconds),
which is what `setup-env.sh` runs.
On a Linux cluster where Python is mounted from NFS, this reduces
`setup-env.sh` source time from ~1s to .75s.
Note: This issue will be eliminated if we upgrade to a newer `jsonschema`
(we'd need to drop Python 2.6 for that). See
https://github.com/Julian/jsonschema/pull/388.
