README: prefer pip3 instead of pip
I tried to run pip command on Debian Bullseye and it failed with following output:
Collecting pytest>=5.0 (from -r requirements.txt (line 1))
Could not find a version that satisfies the requirement pytest>=5.0 (from -r requirements.txt (line 1)) (from versions: 2.0.0, 2.0.1, 2.0.2, 2.0.3, 2.1.0, 2.1.1, 2.1.2, 2.1.3, 2.2.0, 2.2.1, 2.2.2, 2.2.3, 2.2.4, 2.3.0, 2.3.1, 2.3.2, 2.3.3, 2.3.4, 2.3.5, 2.4.0, 2.4.1, 2.4.2, 2.5.0, 2.5.1, 2.5.2, 2.6.0, 2.6.1, 2.6.2, 2.6.3, 2.6.4, 2.7.0, 2.7.1, 2.7.2, 2.7.3, 2.8.0, 2.8.1, 2.8.2, 2.8.3, 2.8.4, 2.8.5, 2.8.6, 2.8.7, 2.9.0, 2.9.1, 2.9.2, 3.0.0, 3.0.1, 3.0.2, 3.0.3, 3.0.4, 3.0.5, 3.0.6, 3.0.7, 3.1.0, 3.1.1, 3.1.2, 3.1.3, 3.2.0, 3.2.1, 3.2.2, 3.2.3, 3.2.4, 3.2.5, 3.3.0, 3.3.1, 3.3.2, 3.4.0, 3.4.1, 3.4.2, 3.5.0, 3.5.1, 3.6.0, 3.6.1, 3.6.2, 3.6.3, 3.6.4, 3.7.0, 3.7.1, 3.7.2, 3.7.3, 3.7.4, 3.8.0, 3.8.1, 3.8.2, 3.9.1, 3.9.2, 3.9.3, 3.10.0, 3.10.1, 4.0.0, 4.0.1, 4.0.2, 4.1.0, 4.1.1, 4.2.0, 4.2.1, 4.3.0, 4.3.1, 4.4.0, 4.4.1, 4.4.2, 4.5.0, 4.6.0, 4.6.1, 4.6.2, 4.6.3, 4.6.4, 4.6.5, 4.6.6, 4.6.7, 4.6.8, 4.6.9, 4.6.10, 4.6.11)
No matching distribution found for pytest>=5.0 (from -r requirements.txt (line 1))