I am not sure if it is python's issue (correct me if this is wrong place for report). But sometimes (probably every 3-4 attempt) when I run script, interpreter just stucks for some time (0.5-3 minutes) and then actually runs the script normally.

To figure out what happens I increased verbosity (python -vv) and actually python hangs on import on this line:

...
# trying /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/hazmat/primitives/kdf/scrypt.abi3.so
# trying /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/hazmat/primitives/kdf/scrypt.so
# trying /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/hazmat/primitives/kdf/scrypt.py
# /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/hazmat/primitives/kdf/__pycache__/scrypt.cpython-37.pyc matches /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/hazmat/primitives/kdf/scrypt.py
# code object from '/home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/hazmat/primitives/kdf/__pycache__/scrypt.cpython-37.(pyc'
(here >>>>>) import 'cryptography.hazmat.primitives.kdf.scrypt' # <_frozen_importlib_external.SourceFileLoader object at 0x7f5c6e3c73c8>
import 'cryptography.hazmat.backends.openssl.backend' # <_frozen_importlib_external.SourceFileLoader object at 0x7f5c6c129c88>
import 'cryptography.hazmat.backends.openssl' # <_frozen_importlib_external.SourceFileLoader object at 0x7f5c6d30d240>
# trying /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/x509/UnsupportedExtension.cpython-37m-x86_64-linux-gnu.so
# trying /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/x509/UnsupportedExtension.abi3.so
# trying /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/x509/UnsupportedExtension.so
# trying /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/x509/UnsupportedExtension.py
# trying /home/2kg/spider/venv/lib/python3.7/site-packages/cryptography/x509/UnsupportedExtension.pyc
...

I had version 3.5 installed before and didn't notice such issue. Other scripts from my project suffer from this symptom as well.

My configuration is:
debian 9
python 3.7.3 [GCC 6.3.0 20170516] on linux
cryptography module – version 2.6.1