Improve pixel density logic in OcrdExif

[bertsky]

#256, #37, OCR-D/ocrd_tesserocr#54

[bertsky]

Looking into Travis failure now... (I get IFDRational instead of integer for xResolution, which is not allowed in the multiplication, at least in Python 3.5 3.4)

[bertsky]

It's strange that IFDRational would not be an allowed operand type for * in Python 3.5 3.4 and Python 3.7 ...

[bertsky]

The Python 3.7-dev test seems to broken already.

[kba]

Yeah, the 3.7-dev failure is unrelated, though strange.

pytest.py: error: unrecognized arguments: --duration=10

Version problem with the tooling it seems.

[bertsky]

Python 3.4 has been removed from support in Pillow >= 6.0:

Drop support for Python 3.4 #3596 [hugovk]

But we get the issue with 5.4.1 according to Travis' log

[bertsky]

No GH issues for the IFDRational problem in Pillow yet. I will try the Pillow selftest in a 3.4 installation...

[kba]

Or we just stop supporting 3.4. IIRC the reason we did support it was because of CentOS still shipping it a year ago. Development stopped, there will be no more 3.4 releases. @cneud @wrznr what do you think?

[codecov-io]

Codecov Report

Merging #257 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
+ Coverage   99.45%   99.45%   +<.01%     
==========================================
  Files          30       30              
  Lines        1277     1279       +2     
  Branches      248      248              
==========================================
+ Hits         1270     1272       +2     
  Misses          6        6              
  Partials        1        1

Impacted Files	Coverage Δ
ocrd_models/ocrd_models/ocrd_exif.py	100% <100%> (ø)	⬆️

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