tests: make the testsuite Python 3.7 compatible

In Python 3.7, the re module changes the name of its _pattern_type
attribute to "Pattern", this patch handles both versions
4 jobs for ph/fix-testsuite-python37 in 167 minutes and 17 seconds (queued for 29 seconds)
latest
Name Stage Failure
failed
archlinux:clang:debug Test
make[1]: *** [Makefile:4388: check] Error 2
make[1]: Leaving directory '/builds/vcsn/vcsn/build'
make: *** [Makefile:5183: installcheck-local] Error 2
Uploading artifacts...
build/*.log: found 1 matching files
build/*/*.log: found 1 matching files

build/*/*/*.log: found 300 matching files

Uploading artifacts to coordinator... ok
id=58464 responseStatus=201 Created token=FkK7pJEJ
ERROR: Job failed: exit code 1
failed
archlinux:gcc:speed Test
make[1]: *** [Makefile:4385: check-am] Error 2
make[1]: Leaving directory '/builds/vcsn/vcsn/build'
make: *** [Makefile:4388: check] Error 2
Uploading artifacts...
build/*.log: found 1 matching files
build/*/*.log: found 1 matching files

build/*/*/*.log: found 300 matching files

Uploading artifacts to coordinator... ok
id=58463 responseStatus=201 Created token=LKrXsH32
ERROR: Job failed: exit code 1