avoid legacy compilation failures on recent ubuntu machines that only have python3-config

lib/python/Makefile.lammps

@@ -1,7 +1,14 @@
 # Settings that the LAMMPS build will import when this package library is used
 # See the README file for more explanation
-
-python_SYSINC = $(shell which python-config > /dev/null 2>&1 && python-config --includes || :)
-python_SYSLIB = $(shell which python-config > /dev/null 2>&1 && python-config --ldflags --embed > /dev/null 2>&1 && python-config --ldflags --embed || (which python-config > /dev/null 2>&1 && python-config --ldflags || :) )
-python_SYSPATH =
+ifeq ($(shell type python3 >/dev/null 2>&1; echo $$?), 0)
+PYTHON=python3
+PYTHONCONFIG = python3-config
+else
+PYTHONCONFIG = python-config
 PYTHON=python
+endif
+
+
+python_SYSINC = $(shell which $(PYTHONCONFIG) > /dev/null 2>&1 && $(PYTHONCONFIG) --includes || :)
+python_SYSLIB = $(shell which $(PYTHONCONFIG) > /dev/null 2>&1 && $(PYTHONCONFIG) --ldflags --embed > /dev/null 2>&1 && $(PYTHONCONFIG) --ldflags --embed || (which $(PYTHONCONFIG) > /dev/null 2>&1 && $(PYTHONCONFIG) --ldflags || :) )
+python_SYSPATH =

tools/singularity/ubuntu20.04.def

@@ -54,6 +54,7 @@ From: ubuntu:20.04
         python3-setuptools \
         python3-virtualenv \
         python3-venv \
+        python-is-python3 \
         rsync \
         ssh \
         texlive \

tools/singularity/ubuntu20.04_amd_rocm.def

@@ -83,6 +83,7 @@ From: ubuntu:20.04
         python3-setuptools \
         python3-virtualenv \
         python3-venv \
+        python-is-python3 \
         rsync \
         ssh \
         vim-nox \

tools/singularity/ubuntu20.04_gpu.def

@@ -88,6 +88,7 @@ From: ubuntu:20.04
         python3-setuptools \
         python3-virtualenv \
         python3-venv \
+        python-is-python3 \
         rsync \
         ssh \
         vim-nox \

tools/singularity/ubuntu20.04_intel_opencl.def

@@ -53,6 +53,7 @@ From: ubuntu:20.04
         python3-setuptools \
         python3-virtualenv \
         python3-venv \
+        python-is-python3 \
         rsync \
         ssh \
         vim-nox \

tools/singularity/ubuntu20.04_nvidia.def

@@ -56,6 +56,7 @@ From: nvidia/cuda:11.6.2-devel-ubuntu20.04
         python3-setuptools \
         python3-virtualenv \
         python3-venv \
+        python-is-python3 \
         rsync \
         ssh \
         vim-nox \

tools/singularity/ubuntu20.04_oneapi.def

@@ -52,6 +52,7 @@ From: ubuntu:20.04
         python3-setuptools \
         python3-virtualenv \
         python3-venv \
+        python-is-python3 \
         rsync \
         ssh \
         texlive \

tools/singularity/ubuntu22.04.def

@@ -54,6 +54,7 @@ From: ubuntu:22.04
         python3-setuptools \
         python3-virtualenv \
         python3-venv \
+        python-is-python3 \
         rsync \
         ssh \
         texlive \