Do not require a specific version of pyyaml

We don't know the exact minimum version of the pyyaml dependency, but
the Ubuntu 16.04 distro default should work fine; don't specify any
version until we run into problems with missing features, which will
give us a strong signal regarding the right minimum version (same
approach as taken for all other Python dependencies).

Signed-off-by: Philipp Wagner <phw@lowrisc.org>

python-requirements.txt

diff --git a/python-requirements.txt b/python-requirements.txt
index 825c325..ab3af74 100644
--- a/python-requirements.txt
+++ b/python-requirements.txt
@@ -19,7 +19,7 @@
 pygments
 pytest
 pytest-timeout
-pyyaml >= 5.1
+pyyaml
 tabulate
 yapf