Fix context output handling for doctests

[twmr]

If the linenumber of a failing doctest is < 9, the failure/error report is wrong.

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[coveralls]

Coverage remained the same at 92.244% when pulling 2f993af on thisch:doctest_lineno into 111d640 on pytest-dev:master.

[RonnyPfannschmidt]

failures are unrelated, thanks 👍