make rpm import optional in koji/__init__.py

Fixes: https://pagure.io/koji/issue/1772

koji/__init__.py

@@ -68,7 +68,10 @@ try:
     import requests_kerberos
 except ImportError:  # pragma: no cover
     requests_kerberos = None
-import rpm
+try:
+    import rpm
+except ImportError:
+    rpm = None
 import shutil
 import signal
 import socket
@@ -605,6 +608,8 @@ class RawHeader(object):
     # see Maximum RPM Appendix A: Format of the RPM File
 
     def __init__(self, data):
+        if rpm is None:
+            raise koji.GenericError("rpm's python bindings are not installed")
         if data[0:3] != RPM_HEADER_MAGIC:
             raise GenericError("Invalid rpm header: bad magic: %r" % (data[0:3],))
         self.header = data
@@ -889,6 +894,8 @@ def splice_rpm_sighdr(sighdr, src, dst=None, bufsize=8192):
 
 def get_rpm_header(f, ts=None):
     """Return the rpm header."""
+    if rpm is None:
+        raise koji.GenericError("rpm's python bindings are not installed")
     if ts is None:
         ts = rpm.TransactionSet()
         ts.setVSFlags(rpm._RPMVSF_NOSIGNATURES|rpm._RPMVSF_NODIGESTS)

setup.py

@@ -14,7 +14,7 @@ def get_install_requires():
         'requests-kerberos',
         'six',
         #'libcomps',
-        'rpm-py-installer',
+        #'rpm-py-installer', # it is optional feature
         #'rpm',
     ]
     if sys.version_info[0] < 3: