Use pip-compile

requirements.in

+bs4
+cryptography
+defusedxml
+requests[security]

requirements.txt

-bs4==0.0.1
-cryptography==2.8
-lxml==4.4.2
-requests[security]==2.22.0
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile
+#
+beautifulsoup4==4.9.3     # via bs4
+bs4==0.0.1                # via -r requirements.in
+certifi==2020.6.20        # via requests
+cffi==1.14.3              # via cryptography
+chardet==3.0.4            # via requests
+cryptography==3.1.1       # via -r requirements.in, pyopenssl, requests
+defusedxml==0.6.0         # via -r requirements.in
+idna==2.10                # via requests
+pycparser==2.20           # via cffi
+pyopenssl==19.1.0         # via requests
+requests[security]==2.24.0  # via -r requirements.in
+six==1.15.0               # via cryptography, pyopenssl
+soupsieve==2.0.1          # via beautifulsoup4
+urllib3==1.25.11          # via requests