--skip-schemas isn't stopping cwltool from trying to connect to the schema server

## Expected Behavior
When I use `cwltool --skip-schemas` on a system with no internet access, the workflow should still be runnable.

## Actual Behavior
When I use `cwltool --skip-schemas` the process hangs at the `Resolved [workflow] to [path to workflow]` step. I have to hard-kill the process to stop it. When I delete all the schema information in my script though (`$schemas` and the `format` instructions which depend on these), the script works as expected.

The code below (provided by @mr-c) replicates this problem for systems which have internet access.

## Workflow Code
```
wget https://github.com/bioexcel/biobb_adapters/raw/master/biobb_adapters/cwl/biobb_md/mdrun.cwl
HTTPS_PROXY=http://127.0.0.5 cwltool --debug --skip-schemas --validate mdrun.cwl

```

## Full Traceback
```
(cwl) 192:test mbessdl2$ HTTPS_PROXY=http://127.0.0.5 cwltool --debug --skip-schemas --validate mdrun.cwl
INFO /Users/mbessdl2/anaconda3/envs/cwl/bin/cwltool 3.0.20200807132242
INFO Resolved 'mdrun.cwl' to 'file:///Users/mbessdl2/work/manchester/BioExcel/workflows_to_adapt/biobb_hpc_cwl_md_list/test/mdrun.cwl'
^CTraceback (most recent call last):
File "/Users/mbessdl2/anaconda3/envs/cwl/bin/cwltool", line 11, in
sys.exit(run())
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/cwltool/main.py", line 1267, in run
sys.exit(main(*args, **kwargs))
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/cwltool/main.py", line 939, in main
loadingContext, uri = resolve_and_validate_document(
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/cwltool/load_tool.py", line 378, in resolve_and_validate_document
document_loader.resolve_all(workflowobj, fileuri)
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/schema_salad/ref_resolver.py", line 957, in resolve_all
loader.validate_links(
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/schema_salad/ref_resolver.py", line 1107, in validate_links
document[d] = self.validate_link(
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/schema_salad/ref_resolver.py", line 1063, in validate_link
link[n] = self.validate_link(field, i, docid, all_doc_ids)
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/schema_salad/ref_resolver.py", line 1053, in validate_link
elif not self.check_exists(link):
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/schema_salad/fetcher.py", line 93, in check_exists
resp = self.session.head(url)
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/requests/sessions.py", line 565, in head
return self.request('HEAD', url, **kwargs)
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/requests/sessions.py", line 530, in request
resp = self.send(prep, **send_kwargs)
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/requests/sessions.py", line 643, in send
r = adapter.send(request, **kwargs)
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/cachecontrol/adapter.py", line 47, in send
resp = super(CacheControlAdapter, self).send(request, **kw)
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/requests/adapters.py", line 439, in send
resp = conn.urlopen(
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/urllib3/connectionpool.py", line 667, in urlopen
self._prepare_proxy(conn)
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/urllib3/connectionpool.py", line 932, in _prepare_proxy
conn.connect()
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/urllib3/connection.py", line 309, in connect
conn = self._new_conn()
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/urllib3/connection.py", line 159, in _new_conn
conn = connection.create_connection(
File "/Users/mbessdl2/anaconda3/envs/cwl/lib/python3.8/site-packages/urllib3/util/connection.py", line 74, in create_connection
sock.connect(sa)
KeyboardInterrupt
```

## Your Environment
* cwltool version: 3.0.20200807132242
(however @mr-c has checked this on more recent versions of cwltool, and it is still an issue for these too)