Remove unneeded boost dependencies from python bindings

[nvmkuruc]

Description of Change(s)

Several usages of boost in the python bindings can be easily removed.

• Remove unused header references to boost/functional/hash.hpp
• Introduce a simple pair and range-like struct for iterators to remove boost::begin and boost::end in favor of std::begin and std::end
• Replace boost::lexical_cast with TfStringify

Fixes Issue(s)

N/A

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8480

[crevilla]

I feel like this would be simpler if we just change the PluginNames function to no longer be templated and just take two const PlugPluginPtrVector::iterator & parameters instead for begin and end. This especially given it's only called in two places.

[nvmkuruc]

Done!