When the 189 patch trailer is not in the spec, don't add it

In here, https://src.fedoraproject.org/rpms/python3.6/c/ca58751035496b75ebc1a5dd46ebdbecca825a14?branch=rawhide @frenzymadness removed:

```patch
-# The following versions of setuptools/pip are bundled when this patch is not applied.
-# The versions are written in Lib/ensurepip/__init__.py, this patch removes them.
-# When the bundled setuptools/pip wheel is updated, the patch no longer applies cleanly.
-# In such cases, the patch needs to be amended and the versions updated here:
-%global pip_version 18.1
-%global setuptools_version 40.6.2
```

At that time, I have not considered the removal a problem. It made sense.

However, now @stratakis adds a new patch to python3.6 and improtpatches add this blurb back.

Maybe importpatches could skip adding this blurb when:

- patch 189 is unchanged
- the blurb was not there before improtpatches run

WDYT?