Fix requires of EPEL rpm

[nforro]

rpmbuild on EPEL doesn't support dynamic buildrequires, but runtime requires are still generated from setup.cfg, so rpm-py-installer has to be substituted with rpm.

[softwarefactory-project-zuul]

Build succeeded.

✔️ pre-commit SUCCESS in 1m 44s
✔️ specfile-tests-rpm-deps SUCCESS in 1m 11s
✔️ specfile-tests-pip-deps SUCCESS in 1m 13s

[nforro]

Requires of the latest rpm:

python(abi) = 3.6
python3.6dist(arrow)
python3.6dist(importlib-metadata)
python3.6dist(rpm-py-installer)
python3.6dist(typing-extensions)
rpmlib(CompressedFileNames) <= 3.0.4-1
rpmlib(FileDigests) <= 4.6.0-1
rpmlib(PartialHardlinkSets) <= 4.0.4-1
rpmlib(PayloadFilesHavePrefix) <= 4.0-1
rpmlib(PayloadIsXz) <= 5.2-1

Requires of the rpm with this fix applied:

python(abi) = 3.6
python3.6dist(arrow)
python3.6dist(importlib-metadata)
python3.6dist(rpm)
python3.6dist(typing-extensions)
rpmlib(CompressedFileNames) <= 3.0.4-1
rpmlib(FileDigests) <= 4.6.0-1
rpmlib(PartialHardlinkSets) <= 4.0.4-1
rpmlib(PayloadFilesHavePrefix) <= 4.0-1
rpmlib(PayloadIsXz) <= 5.2-1

[softwarefactory-project-zuul]

Build succeeded (gate pipeline).

✔️ pre-commit SUCCESS in 1m 49s