Changed pxi bitmask enum base type to enum.IntFlag

[tkrebes]

• [x] This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

• Removes BitmaskProperty as Python 3.6 and later have support for IntFlag
• Changes pxi bitmask enums base type to enum.IntFlag
• Updates nisyscfg API to treat BaseFlag just like BaseEnum

Why should this Pull Request be merged?

Improves the usability of bitfield enum base type with IntFlag

What testing has been done?

Checked that properties worked with a PXIe-1095 chassis

>>> import nisyscfg
>>> session = nisyscfg.Session()
>>> hw = list(session.find_hardware())
>>> hw[6]
HardwareResource(name=PXIChassis1)
>>> hw[6].pxi.fan_mode
<FanModes.AUTO: 1>
>>> hw[6].pxi.supported_fan_modes
<FanModes.HIGH|SAFE_MANUAL|AUTO: 7>
>>> hw[6].pxi.supported_cooling_profiles
<CoolingProfiles.WATTS_82|WATTS_58|WATTS_38: 7>
>>> hw[6].pxi.supported_inhibit_modes
<InhibitModes.MANUAL|DEFAULT: 3>
>>> hw[6].pxi.cooling_profile
<CoolingProfiles.WATTS_82: 4>