requirements.txt: Move the six dependency into the general requirements

config_template.py depends on six, which isn't listed in the default
requirements.txt. This previously frequently wasn't a problem, because
six used to be a standard package being installed into a venv, and
lots of other projects depended on it.

It also does get installed for unit and integration tests via
tests/requirements.txt, so any broken dependency on six wouldn't be
detected by tox runs.

However, as other projects and distributions have phased out Python
2.7 support the dependency on six becomes less common. Thus, as long
as ceph-ansible does require it for config_template.py, add it to the
base requirements.

Signed-off-by: Florian Haas <florian@citynetwork.eu>

requirements.txt

@@ -1,3 +1,4 @@
 # These are Python requirements needed to run ceph-ansible master
 ansible>=2.9,<2.10,!=2.9.10
 netaddr
+six

tests/requirements.txt

@@ -1,5 +1,4 @@
 # These are Python requirements needed to run the functional tests
-six==1.10.0
 testinfra>=3,<4
 pytest-xdist==1.28.0
 pytest>=4.6,<5.0