Docs: Specify the version requirement of Python3

This requirement is coming from the 'imgtool' Python package,
which is used to sign the binaries when BL2 is enabled.

Change-Id: I767939dc393c988801d4799d69d94c1fd9c5f21e
Signed-off-by: David Vincze <david.vincze@linaro.org>
diff --git a/docs/getting_started/tfm_sw_requirement.rst b/docs/getting_started/tfm_sw_requirement.rst
index 673e4e7..24d42aa 100644
--- a/docs/getting_started/tfm_sw_requirement.rst
+++ b/docs/getting_started/tfm_sw_requirement.rst
@@ -153,7 +153,7 @@
 - Git tools v2.10.0
 - CMake (see the "Supported CMake versions" chapter)
 - GNU Make (see the "Supported make versions" chapter)
-- Python3 and the pip package manager (from Python 3.4 it's included)
+- Python3 (3.6 or newer)
 - Python3 packages: *cryptography, pyasn1, yaml, jinja2 v2.10, cbor v1.0.0, click, imgtool v1.6.0*
 
   .. code-block:: bash
@@ -212,8 +212,7 @@
 - GNU make should be installed by selecting appropriate package during
   cygwin
   installation.
-- Python3 `(native Windows version) <https://www.python.org/downloads/>`__ and
-  the pip package manager (from Python 3.4 it's included)
+- Python3 (3.6 or newer) `(native Windows version) <https://www.python.org/downloads/>`__
 - Python3 packages: *cryptography, pyasn1, yaml, jinja2 v2.10, cbor v1.0.0, click imgtool v1.6.0*
 
   .. code-block:: bash