Find python3 in cmake, fix warning

[scpeters]

There are some cmake warnings now reported by jenkins since we upgraded one of the jenkins plugins:

• https://build.osrfoundation.org/job/sdformat-ci-sdformat9-bionic-amd64/31/

CMake Warning at CMakeLists.txt:226 (message):
  -- 	Python psutil package not found.  Memory leak tests will be skipped

The issue is that we are installing the python3 version of psutil, so we need to find the python3 interpreter in cmake.

[mjcarroll]

The sdformat ci is still failing one test, which seems psutil related:

81: [ RUN      ] ElementMemoryLeak.SDFCreateDestroy
81: Traceback (most recent call last):
81:   File "/var/lib/jenkins/workspace/sdformat-ci-pr_any-ubuntu_auto-amd64/sdformat/tools/get_mem_info.py", line 4, in <module>
81:     import psutil;
81: ImportError: No module named psutil
81: unknown file: Failure
81: C++ exception with description "stoi" thrown in the test body.

[codecov-commenter]

Codecov Report

Merging #328 into sdf9 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             sdf9     #328   +/-   ##
=======================================
  Coverage   86.40%   86.40%           
=======================================
  Files          59       59           
  Lines        9053     9053           
=======================================
  Hits         7822     7822           
  Misses       1231     1231           

---

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[scpeters]

The sdformat ci is still failing one test, which seems psutil related:

should be fixed by 1f84744

[scpeters]

CI is 🟢

[azeey]

LGTM! I guess CMake warnings are ignored in windows?

[scpeters]

LGTM! I guess CMake warnings are ignored in windows?

I guess not; it actually has quite a few cmake warnings, but we'll just leave those alone for now