It looks like you don't have Łukasz key and your GnuPG is not configured for automatic key download.

Automatic key download works for me:

$ gpg --verify Python-3.8.0b4.tgz.asc 
gpg: assuming signed data in 'Python-3.8.0b4.tgz'
gpg: Signature made 2019-08-30T00:43:07 CEST
gpg:                using RSA key E3FF2839C048B25C084DEBE9B26995E310250568
gpg: requesting key 0xB26995E310250568 from hkp server keys.fedoraproject.org
gpg: key 0xB26995E310250568: public key "Łukasz Langa (GPG langa.pl) <lukasz@langa.pl>" imported
gpg: Total number processed: 1
gpg:               imported: 1
gpg: Good signature from "Łukasz Langa (GPG langa.pl) <lukasz@langa.pl>" [undefined]
gpg:                 aka "Łukasz Langa <lukasz@python.org>" [unknown]
gpg:                 aka "Łukasz Langa (Work e-mail account) <ambv@fb.com>" [undefined]
gpg:                 aka "[jpeg image of size 24479]" [unknown]


You could also download the key from keybase:

$ gpg --fetch-keys "https://keybase.io/ambv/pgp_keys.asc?fingerprint=e3ff2839c048b25c084debe9b26995e310250568"
gpg: requesting key from 'https://keybase.io/ambv/pgp_keys.asc?fingerprint=e3ff2839c048b25c084debe9b26995e310250568'
gpg: key 0xB26995E310250568: "Łukasz Langa (GPG langa.pl) <lukasz@langa.pl>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1