Underscore character does not exist for cmr10 font #16995

Bug report

The bundled font cmr10 (and it's associated family) do not contain the underscore character. Instead, the unicode character for the underscore (according to this website anyways) is actually mappted to the overdot.

See this StackOverflow for details, but I'll repeat here.

Code for reproduction

import matplotlib.pyplot as plt
plt.rcParams.update({
            'font.family': 'serif',
            'font.serif': 'cmr10',
})

variable = range(5)
plt.plot(variable, variable, label='test_underscore')
plt.plot(variable, variable, label='escape\_underscore')
plt.plot(variable, variable, label=r'rawtest_underscore')
plt.plot(variable, variable, label=r'rawescape\_underscore')
_=plt.legend()

Actual outcome

Expected outcome

Matplotlib version

• Operating system: Manjaro 19.0.2
• Matplotlib version: 3.1.3
• Matplotlib backend (print(matplotlib.get_backend())): module://ipykernel.pylab.backend_inline
• Python version: 3.7.6

Other
cmr10 loaded into FontForge shows that:
A) the underscore character doesn't exist
B) the normal underscore character (005f) is replaced by the overdot