Fix parsing pofile with obsoletes

[mbirtwell]

Fixes the handling of the unit before the obsolete unit. Previously it would mark the unit before an obsolete unit as obsolete also.

Fixes multi-line msgctxt statements. Previously it would just take the first line.

Some refactoring:

• the transition between finishing one unit and starting the nextis clearer
• separate the processing of keywords and continuation lines
• combine the reset and initialisation code
• Make the handling of strings consistent.
• Add some nascent error handling, removed some errors in test inputs

[codecov-io]

Current coverage is 90.14% (diff: 97.43%)

Merging #452 into master will increase coverage by 0.02%

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+ Hits           3569       3587    +18   
- Misses          391        392     +1   
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[akx]

Out of curiosity -- is the parser still able to read files that start with a newline, i.e. is this just cleanup? :)

[mbirtwell]

So the combination of r and triple quotes means that you can't escape new lines using "\". The original actually tested the string:

\
# Translations template for AReallyReallyLongNameForAProject.
...

My version:

# Translations template for AReallyReallyLongNameForAProject.
...

So in fact the test is now testing handling files which start with a newline. It looks like the original author intended there to be no blank line at the start of the file, but introduced an invalid character instead. The parser now prints a warning for lines it doesn't understand, so I removed the character to prevent that warning getting printed during unit tests.

[akx]

Fair enough! Thank you for the explanation.

[akx]

Could you fold the Python 2.6 changes into the relevant earlier commits?

Otherwise this looks nice :)