TO Postinstall: Check if hashlib.scrypt() exists before calling it

CHANGELOG.md

@@ -19,6 +19,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - [#6255](https://github.com/apache/trafficcontrol/issues/6255) - Unreadable Prod Mode CDN Notifications in Traffic Portal
 - [#6259](https://github.com/apache/trafficcontrol/issues/6259) - Traffic Portal No Longer Allows Spaces in Server Object "Router Port Name"
 - [#6175](https://github.com/apache/trafficcontrol/issues/6175) - POST request to /api/4.0/phys_locations accepts mismatch values for regionName.
+- [#6283](https://github.com/apache/trafficcontrol/issues/6283) - The Traffic Ops Postinstall script will work in CentOS 7, even if Python 3 is installed
 
 ### Changed
 - Updated `t3c` to request less unnecessary deliveryservice-server assignment and invalidation jobs data via new query params supported by Traffic Ops

traffic_ops/install/bin/_postinstall.py

@@ -429,7 +429,7 @@ def hash_pass(passwd): # type: (str) -> str
 	p_val = 1
 	dklen = 64
 	salt = os.urandom(dklen)
-	if sys.version_info.major >= 3:
+	if sys.version_info.major >= 3 and hasattr(hashlib, 'scrypt'): # Python 2.7 and CentOS 7's Python 3.6 do not include hashlib.scrypt()
 		hashed = hashlib.scrypt(passwd.encode(), salt=salt, n=n, r=r_val, p=p_val, dklen=dklen)
 	else:
 		hashed = Scrypt(password=passwd.encode(), salt=salt, cost_factor=n, block_size_factor=r_val, parallelization_factor=p_val, key_length=dklen).derive()