Rewrite CMake configuration in "modern CMake"

Rewrite of the CMake configuration along the lines of LHCb!2931 (merged) (see there for more details).

Note: also update version to 53.0 and constraint on Phys version

Hlt/Hlt1Conf/CMakeLists.txt

 ###############################################################################
-# (c) Copyright 2000-2018 CERN for the benefit of the LHCb Collaboration      #
+# (c) Copyright 2000-2021 CERN for the benefit of the LHCb Collaboration      #
 #                                                                             #
 # This software is distributed under the terms of the GNU General Public      #
 # Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   #
@@ -8,13 +8,12 @@
 # granted to it by virtue of its status as an Intergovernmental Organization  #
 # or submit itself to any jurisdiction.                                       #
 ###############################################################################
-gaudi_subdir(Hlt1Conf)
+#[=======================================================================[.rst:
+Hlt/Hlt1Conf
+------------
+#]=======================================================================]
 
-find_package(pytools)
+gaudi_install(PYTHON)
 
-gaudi_install_python_modules()
-
-gaudi_add_test(pytests
-               COMMAND python -m pytest -v --doctest-modules
-                       ${CMAKE_CURRENT_SOURCE_DIR}/python)
-gaudi_add_test(QMTest QMTEST)
+gaudi_add_tests(pytest ${CMAKE_CURRENT_SOURCE_DIR}/python)
+gaudi_add_tests(QMTest)