Declare which Python packages we use

Add pip requirements files. We'll have separate requirements files for
different target audiences. Each file can use `-r` lines to include other
files.

This commit adds two requirement files: one with everything that's needed to
pass the CI, and one with additional tools that are suggested for Mbed TLS
maintainers to install locally.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

scripts/ci.requirements.txt

@@ -0,0 +1,10 @@
+# Python package requirements for Mbed TLS testing.
+
+# Use a known version of Pylint, because new versions tend to add warnings
+# that could start rejecting our code.
+# 2.4.4 is the version in Ubuntu 20.04. It supports Python >=3.5.
+pylint == 2.4.4
+
+# Use the earliest version of mypy that works with our code base.
+# See https://github.com/ARMmbed/mbedtls/pull/3953 .
+mypy >= 0.780

scripts/maintainer.requirements.txt

@@ -0,0 +1,6 @@
+# Python packages that are only useful to Mbed TLS maintainers.
+
+-r ci.requirements.txt
+
+# For source code analyses
+clang