Tried pip3 in verbose mode install : pip3 install . --root=%installroot% -v, but still not clear why it fails.
Collecting ruamel.yaml.clib>=0.1.2 (from ruamel.yaml==0.16.10)
1 location(s) to search for versions of ruamel.yaml.clib:
* https://pypi.org/simple/ruamel-yaml-clib/
Getting page https://pypi.org/simple/ruamel-yaml-clib/
Found index url https://pypi.org/simple
Looking up "https://pypi.org/simple/ruamel-yaml-clib/" in the cache
Request header has "max_age" as 0, cache bypassed
Starting new HTTPS connection (1): pypi.org:443
Incremented Retry for (url='/simple/ruamel-yaml-clib/'): Retry(total=4, connect=None, read=None, redirect=None, status=None)
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f8872ddef10>: Failed to establish a new connection: [Errno -2] Name or service not known')': /simple/ruamel-yaml-clib/