The feature use_coverage of hypothesis caused the failing tests, because
the printer_file_test is sensitive to the coverage analysis of
hypothesis.
Fixed by disabling use_coverage for the crashing tests
hypothesis introduced a regression in 3.29.0 (or at least changed
behaviour). Until I have found the problem I will pin it to the last
working version.
Also two tests should be prevented from failing when they are slow.
The file-printer did not automatically flush and thus behaved
differently to the other printer-classes.
Now the default behaviour is to flush after every call of _raw(). This
can be disabled by calling the file-printer with auto_flush=False.
fixes#106
