Fix CSS lookup for simulator server

[KevinWMatthews]

Fix CSS lookup when running pymodbus.simulator.

Currently, the pymodbus simulator fails to load CSS:

Before

With this change, I observe CSS loading:

After

Background

I can confirm that I observed a 404 on the pymodbus_styles.css:

pymodbus_styles.css:1  Failed to load resource: the server responded with a status of 404 (Not Found)
pymodbus_styles.css:1  Failed to load resource: the server responded with a status of 404 (Not Found)

in both Chrome and Firefox.

I suspect that this regex will only serve files that contain the characters:

• a-z (lowercase letters)
• 0-9 (digits)
• . (any number of)

I did confirm that an alternative solution would work:

• rename all pymodbus_styles.css references to pymodbusstyles.css
• rename the file itself to pymodbusstyles.css

This removes the _, allowing the existing regex to work. However, I proposed what I feel is a more light-weight solution: extending the regex.

[janiversen]

LGTM, thanks.