Relax dependency version requirements

It simplifies dependency managements.

Example: PyYAML 5.4.1 (most recent at the moment of writting this) is
available for Python 3.6+. However we should keep 5.3.1 for Python 3.5.
Anyway, it would be nice to depend on a newer (but compatible) version
on newer python versions to bring bugfixes and security updates. See
more in [1].

[1]: #282

Supersedes PR #282.

Author: Totktonada

requirements.txt

@@ -1,2 +1,2 @@
-PyYAML==5.1
-gevent==21.1.2
+PyYAML==5.*
+gevent==21.*