README: require python >= 3.7, clean up module descriptions

libpython was added in 2cc86f094a, it seems
because of python-systemd module that we built. But libpython by itself
is not enough for actual python programs, and now we also list python itself,
so let's drop libpython from the list.

meson requires >= 3.7. We have CI that runs on CentOS8 with Python 3.6, but
let's not provide official support for an EOL Python version. Individual
distributions can provide backports, but we don't need to mention that in
the user-facing docs. According to [1], 3.7 is on life support and 3.6 is EOL.

[1] https://devguide.python.org/versions/

README

@@ -195,7 +195,6 @@ REQUIREMENTS:
         libgcrypt (optional)
         libqrencode (optional)
         libmicrohttpd (optional)
-        libpython (optional)
         libidn2 or libidn (optional)
         gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
         openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
@@ -206,17 +205,17 @@ REQUIREMENTS:
         gperf
         docbook-xsl (optional, required for documentation)
         xsltproc    (optional, required for documentation)
+        python >= 3.7 (required by meson too, >= 3.9 is required for ukify)
         python-jinja2
-        python-pefile
+        python-pefile (optional, required for ukify)
         python-lxml (optional, required to build the indices)
-        python >= 3.5
+        pyelftools (optional, required for systemd-boot)
         meson >= 0.53.2
         ninja
         gcc >= 4.7
         awk, sed, grep, and similar tools
         clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
                 from source code in C)
-        pyelftools (optional, required for systemd-boot)
 
         During runtime, you need the following additional
         dependencies:
@@ -270,8 +269,9 @@ REQUIREMENTS:
         Additional packages are necessary to run some tests:
         - busybox            (used by test/TEST-13-NSPAWN-SMOKE)
         - nc                 (used by test/TEST-12-ISSUE-3171)
-        - python3-pyparsing
-        - python3-evdev      (used by hwdb parsing tests)
+        - python             (test-udev which is installed is in python)
+        - python-pyparsing
+        - python-evdev       (used by hwdb parsing tests)
         - strace             (used by test/test-functions)
         - capsh              (optional, used by test-execute)