Better parsing of README files in RST format

In our Pypi packages, we often have README files in RST format. pyp2rpm seems to simply copy the first few line of the README file into the generated RPM file, without parsing it to eliminate RST comments.

For an example, look at https://raw.githubusercontent.com/pywbem/pywbem/master/README_PYPI.rst. Here is what pyp2rpm generates from that:

```
%description
.. README file for Pypi.. Note: On Pypi, variable substitution with raw content
is not enabled, so .. we have to specify the package version directly in the
links... begin of customization for the current version .. |pywbem-version-mn|
replace:: 0.17 .. _Readme file on GitHub: .. _Documentation on RTD: .. _Change
log on RTD: .. end of customization for the current versionPywbem is a WBEM...
```
It does not even get beyond the comment section.

Proposal:
- An easy first stage would be to simply omit RST comment lines.
- Better would be to properly render the RST file into a text file and use that.