Fix tox.ini to be compatible with latest version

[martin-majlis-s1]

When I was following README.md how to test scalyr agent, I was not able to finish it for the following reasons:

1. Configuration passenv: Values cannot no longer be separated by spaces - passenv regression when multiple variables are named on the same line tox-dev/tox#2615
2. Configuration whitelist_externals: This config option no longer exists and was replaces with allowlist_externals - Add support for allowlist_externals tox-dev/tox#2134 + Remove deprecated whitelist_externals configuration option tox-dev/tox#2600

When I was trying to commit changes, it has failed since pre-commit was configured to use Python 3.6 - https://devguide.python.org/versions/ - which is no longer supported. So I have bumped it to 3.10.

[github-actions]

Test Results

       33 files  ±0         33 suites  ±0   1h 2m 32s ⏱️ + 3m 40s
  1 437 tests ±0    1 419 ✔️ ±0    18 💤 ±0  0 ❌ ±0 
11 080 runs  ±0  10 758 ✔️ ±0  322 💤 ±0  0 ❌ ±0 

Results for commit 4a9a1a5. ± Comparison against base commit bc78d98.

[ArthurKamalov]

LGTM, thanks.

[codecov]

Codecov Report

Merging #1096 (4a9a1a5) into master (bc78d98) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1096      +/-   ##
==========================================
+ Coverage   79.91%   79.98%   +0.07%     
==========================================
  Files         185      185              
  Lines       42653    42653              
  Branches     4796     4796              
==========================================
+ Hits        34083    34113      +30     
+ Misses       7401     7372      -29     
+ Partials     1169     1168       -1     

Impacted Files	Coverage Δ
...gent/builtin_monitors/kubernetes_events_monitor.py	65.47% <0.00%> (-0.98%)	⬇️
scalyr_agent/builtin_monitors/docker_monitor.py	77.02% <0.00%> (+0.24%)	⬆️
...calyr_agent/builtin_monitors/kubernetes_monitor.py	67.41% <0.00%> (+0.26%)	⬆️
scalyr_agent/monitor_utils/k8s.py	82.27% <0.00%> (+1.28%)	⬆️
scalyr_agent/scalyr_client.py	80.73% <0.00%> (+1.70%)	⬆️