Refer to https://packaging.python.org/guides/making-a-pypi-friendly-readme/

Enable DMTimer PWM pins on BBB and PB

Building with GCC 10.2.1 fails due linking errors caused by having multiple
definitions of the variables defined in the source/constants.h header file.

Fix this by moving the variables definition to the source/constants.c file,
to avoid the variables to be defined each time that the header is included.

While being there, use a single variable instead of having one for each
object that is added to the module.

fix link error due multiple variables definitions

PWM outputs on P9_28 and P9_42 are part of the ECAP hardware block.  This causes 'ecap' to be in the path instead of 'pwm'.  For example:
`/sys/devices/platform/ocp/48300000.epwmss/48300100.ecap/pwm/pwmchip0/pwm0`
is one character longer than:
`/sys/devices/platform/ocp/48302000.epwmss/48302200.pwm/pwm/pwmchip4/pwm0`
Thus the index in pwm_path must be adjusted for the ecap_path_udev path.  This fixes issue #342

Fix pwm output for ecap pins P9_28 and P9_42 #342

PyEval_ThreadsInitialized() and PyEval_InitThreads() are deprecated and break the build, adding a version check to ignore if we are compiling with a newer Python version.

Fix compilation for newer versions of Python