[FIX] UrlReader: shorten TempFile extension

[PrimozGodec]

Issue

It can happen that the file name is too long due to the long suffix of the temp-file (the suffix is complete path joined together with _). It invokes File name too long: 'filename' error.
This is one of the issues causing that tests cannot run on cond-forge where file paths are really long conda-forge/orange3-feedstock#77

Description of changes

Temfile does not get a complete name as a suffix but just an ending which makes sure that the correct reader reads the file.

I do not know what exactly is the reason that complete path was provided as a name so I hope that I didn't break anything.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #4747 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4747      +/-   ##
==========================================
- Coverage   83.80%   83.76%   -0.04%     
==========================================
  Files         280      275       -5     
  Lines       56496    55682     -814     
==========================================
- Hits        47344    46640     -704     
+ Misses       9152     9042     -110